In Baby Cakes, Rarity doesn't even provide an excuse as to why she won't babysit the twins. Though it's easy to figure out what reason exactly: she knows her shop isn't child-safe.
More like she knows that she isn't child-safe, either. Rarity knows that she is highly temperamental, especially when she's at work. She also knows that she only barely has enough restraint to resist strangling her own baby sister when she visits. So the prospect of babysitting two one-month-old infants in her shop during business hours? It's not even a question. NO.
Another reason for this: Rarity mentioned early on about the "magic surges" that come and go with baby unicorns. Since she was probably still living with her parents when Sweetie Belle was born, she had firsthand experience about having to deal with said surges and thus is not yet able to handle that again.
Pinkie's absolute failure at Stand-up. She was using Steven Wright material, which generally requires a more flat, dry delivery - which is a polar opposite to Pinkie at any time.
Pinkie, usually the element of laughter and fun, is seriously crying and feeling helpless while babysitting the twins, mainlydue to the stress of taking care of multiple children.. Bad enough by itself, but this is made even worse when you realize that nopony else, including the Cakes, thought she could handle the job of babysitting the twins, and she was at the breaking point before she realized she had shot herself in the hoof accepting the job. She was probably subconsciously admitting to herself that they might all be right about her being incapable of doing the job. Which made her feel like a completely incompetent failure as a result, and which is probably the real reason for her breakdown.
Weirdly enough, it sort of breaks the aesop of Applebuck Season of "Don't be proud and ask for help if you need it."
It doesn't, really — if Pinkie had followed that aesop and let Twilight help her, there'd be a lot less trouble for her. Declining the help wasn't a good move, since Pinkie didn't know if she really was up to the task. (Of course, the way Twilight phrased her offer, it might have come off as deeply affecting Pinkie's pride.) Luckily for the twins, she could eventually do it. It'd have been a lot worse if she turned out unable to.
But Applejack been doing the job her whole life, so it's only natural that she would be reluctant to accept help for something that she's been doing forever. Pinkie thought she could handle babysitting, but before now, she's never done anything like that and people made it pretty clear they didn't think she was up to the task, so maybe Pinkie just felt as though this was something she had to do, both for herself and her family, essentially. Not to mention, a lot of times people have mistaken Pinkie's Cloudcuckoolander personality for being simple, like in Swarm of the Century, only for her to turn out to be correct.
There has been a lot of speculation about how Pumpkin Cake was able to levitate herself when Twilight, the Element of Magic, couldn't do that in "Sonic Rainboom".
Well...she probably could. It seems to be a simple variation of the levitation spell that ALL unicorns use to move things; Pumpkin simply applied it to herself. Twilight could have levitated herself to Cloudsdale, she just didn't think of it!
Adding to the above: Twilight was looking for flight spells; she didn't think of just levitating.
And why didn't she think of something so simple? Because the levitation spell isn't supposed to be used on yourself. She's too logical to use a spell for anything but its intended purpose. And, of course, it wouldn't have solved the problem of how to get everypony else to Cloudsdale.
There's also the simple fact that Pumpkin probably only weighs a fraction of what a full grown pony does. Levitating a full-grown pony for any amount of time would require much more effort, possibly pushing it into Awesome, but Impractical territory for Twilight and downright impossible for a lesser unicorn. In mechanical terms, Pumpkin's 'Power to Weight Ratio' is better than Twilight's, and that's why she can self levitate.
Actually, this is averted in the season 3 opener when Twilight actually does levitate herself to avoid a collision with the ground at the bottom of the stairs. Logic would dictate that she didn't leviate herself up to cloudsdale because the cloudwalking spell is actually more practical. Magic requires energy, and levitation requires constant focus. So much focus that if she were to break concentration at any time, she would fall through the clouds, and wouldn't even be able to really watch the competition due to this. It also leaves her with no solution for getting her friends up there. As for the use of the balloon, wouldn't you rather take a comfortable ride in a hot air balloon as opposed to using the much needed energy the magic would require to float the hundreds of feet (possibly more) up to the city itself?
This cold start of this episode explains how unicorns, pegasi and earth ponies are all able to bear children that are from one of the other races by having them somewhere in their family history This helps explain what Scootaloo is doing in the classroom in "Family Appreciation Day," and why she's in Ponyville at all instead of Cloudsdale. One or both of her parents can't fly so she has to live on the ground.
This theory would also provide an explanation for unicorns being nobility. They've been interbreeding like most nobles do, not so much out of 'keeping the bloodline pure', but also out of trying to make more unicorns!
Why do the twins come to comfort Pinkie so quickly when she breaks down crying? She's been their constant playmate since they were born. Not to mention that the Cakes have been confirmed by Word of God to pretty much consider Pinkie their daughter. Even if they're babies, they just realized they'd made their best friend and Cool Big Sis cry; of course they'd try to comfort her!
It also explains why they were so rambunctious to begin with; they think of Pinkie as a playmate and not a caretaker and believe it's all a game until she cries and they realize it isn't. Real kids do this as well, assuming that certain people are just there to have fun with all the time.
In addition, Pinkie went to some pains to comfort them when they cried— whenever they cried, as a matter of fact. So they learned that's what you do. You don't leave someone you care about sad and crying. They immediately sought to comfort her with something she technically taught them at first on accident, and then kept using because it was the only thing she could use to get them to cheer up: the bag of flour.
In addition to being their constant playmate as the above posters just mentioned, Pinkie lives in an apartment above the shop, as the Cakes themselves apparently do. The twins haven't just played with Pinkie their whole lives up to this point, she lives with them, unless she had to move out in order to give the babies room. As such, the twins may not be aware Pinkie isn't biologically related to them, so not only do Cup Cake and Carrot Cake think of Pinkie as their daughter, the twins may not realize she isn't related to them. They may believe she is their actual older sister due to their young age. Think about it. They know their parents because they most likely saw them at birth. Who was one of the next ponies they saw, and kept seeing on a near-constant basis? Pinkie. Not that it exactly matters if they think she's really their sister or not. They live with her. This is part of why they would never have expected a time would come where she wouldn't just be playing with them until she snapped and started bawling. She had never been in a caretaker role with them before. They thought she was just playing with them to make them feel better until their parents returned. To be fair, she kind of was doing that because it's what she largely assumed it would work best in keeping them happy in between dealing with the responsibilities laid out for her by their parents.
Besides which, magic Unicorn pony, so comparisons to Real Life horses may not necessarily apply.
Why did the twins immediately start crying when their parents left? Because their parents just left; that's why! Additionally they're unaware of when their parents are coming back so of course they're upset.
On top of that, babies learn very quickly at that age but lack the skills to make logical chain deduction. So they were probably aware enough already to realize that anyone who leaves through the front doors is not necessarily coming back soon...if at all. That's part of why the peek-a-boo game Pinkie was playing is so important to baby development. It helps them learn that "if someone we care about goes away, they will come back sooner or later."
Pinkie's instant desire to play with the Cake babies shortly after their birth seems a bit over the top at first. However if her backstory in the Cutie Mark Chronicles is true, and her childhood was likely not a particularly happy one prior to gaining her Cutie Mark, then Pinkie's efforts are her way of ensuring that her foster siblings have all the laughter and playtime she didn't get growing up.
Rarity refusing to babysit the Cake twins was actually generous of her; as a seamstress, her house is FILLED with needles, scissors, pins, beads, spools, and countless other things hazardous to babies. Combine that with the power of Pumpkin Cake's magic spurts, which Rarity knew was going to start happening soon, and you have a recipe for disaster.
It might seem strange that Mr and Mrs Cake started looking to the rest of the Mane 6 to babysit the Cake Twins before turning to Pinkie, but considering the individual Ponies in question, they all (except Rarity, as it was already explained above) have certain things about them that actually would make them decent babysitters, to clarify;
Fluttershy already gained experience in dealing with kids after 'Stare Master' and is gentle and patient enough that she might've had no trouble dealing with the Cake Twins.
Twilight already had years of experience with kids that comes from her growing up with Spike, who was basically a little brother to her ever since he hatched from his egg years ago. And she already showed in the 'Baby Cakes' episode how she is knowledgeable about how babies should be taken care of (presumably from her said relationship with Spike and/or reading about babies). Of course, she was aware of her own social limitations.
Applejack, in hindsight, likely has the most experience (out of the entire Mane 6) in taking care of baby foals, considering how considering how her parents have been basically confirmed to be dead, she likely has had to help raise Applebloom alongside Big Macintosh and Granny Smith when Applebloom was a baby herself.
Rainbow Dash, though sometimes lazy, has been known to work hard when she has to and sometimes does at least come across as more responsible than Pinkie Pie. And if her own sonic history is any indicator, she'd know exactly what to be on the watch for around Pound.
Why was Mr. Cake so worried at the maternity ward? Because it is so dangerous for a horse to bear twins. He didn't know if Cup would live. This also explains why she didn't appear in Winter Wrap-Up; she didn't want to overtax her body.
Why is Pinkie having so much trouble when she had taken care of two little sibs before? Simple, she had backup in the form of Maud.
So how exactly is Pound Cake able to lift Pinkie and still fly at high speeds dspite only being a month old? Well probably just Rule of Funny... Or maybe there's another reason. we're told that Unicorn foals have magic spurts at young ages and see it for ourselves. It's also possible that baby Pegasi may have flight spurts as well (excplaining why Pound can fly while Scootaloo can't). So therefore, perhaps it's also possible that Earth Ponies have spurts of strength (the main outcome of Earth Poiny magic) at that age too. While Pound has the wings of a Pegasus, both his parents are Earth Ponies, meaning he has some Earth Pony blood in him. He could be getting Strength surges in addition to Flight surges.
On her own character sheet, Pinkie Pie is listed as sanguine. As noted above, quite a number of defects associated with this temperament play out in Technicolor. She subverts the key defect of the temperament by presenting the Cakes' parents with an immaculate house upon return.
At one point we see Pumpkin Cake using her magic to easily walk through the basket that Pinkie Pie had trapped her and Pound under. What if Pumpkin decides to take up a life of crime later? Or becomes a serial killer?!
As mentioned above, we learn that infant unicorns sometimes have powerful magic surges that fade as they grow up. So that probably means that the difficulties that Pinkie had with Pumpkin Cake in this episode are not uncommon for the caregivers of infant unicorns, especially those caregivers who are not unicorns themselves. But now consider the altogether likely possibility that the strength and frequency of an infant unicorn's magic surges are a function of that unicorn's full magical potential as an adult, that is, that a unicorn with very strong natural magical aptitude is likely to have many and powerful surges as an infant. Now imagine what Twilight Sparkle must have been like as an infant. Imagine how difficult it must have been for her parents to care for her while also preventing her from destroying them, herself, or the surrounding city block. Do you think the kind of upbringing Twilight must have had as an infant might have anything to do with her neuroses today?
This brings up some Fridge Brilliance when the Season 2 finale rolls around: if Twilight was uberpowerful as an infant it makes a lot more sense that she'd have an alicorn for a babysitter. It also helps that her brother's special talent is large magical shields.
Also, Twilight is not the only victim, here. Rainbow Dash is also VERY sensitive about her aerobatics, and does everything she can to prove them to herself and the world every chance she gets. She can also fly fast enough to shatter the light spectrum, producing an explosion powerful enough to break rocks and demolish buildings. Her parents may have had to strap Baby Dashie's wings down to keep her from Rainbooming the house apart. (HOWEVER! They never say in the episode if pegasi have bursts of flight, similar to unicorns and magic, or not.)
Though it is shown multiple times that Scootaloo, who is much older than Pound Cake, cannot yet fly, so it is entirely possible that Pound Cake will lose the ability to fly once he starts gaining weight, to only regain it when he gets older.
Unless Scootaloo is such a weak pegasus that she will never be able to fly normally. Maybe a result of genetic instability? We've only ever seen her hovering for a couple of seconds, and she spends all her time in Ponyville with no family in sight...
Another thing is that Pound Cake is one month old and he can already fly to the point that he can speed around a house while carrying a pony several times his size and weight. If this is just a fraction of what he'll be able to do, he might be outdoing Rainbow Dash when he's older. If Baby Dashie had to have her wings tied, ask yourself: What's faster than a sonic rainboom? Now apply that to Pound Cake. Now factor in Pound Cake's name. He already likes to "pound" things. Sooner or later, he's going to figure out that faster speeds equal harder pounding on things. Here's to hoping he never thinks to try pounding the ground at supersonic speed.
At the end of "Baby Cakes", neither twin has a diaper on before the go to sleep. It's more than likely they'll "make lemonade" or "boom-boom" not too long after.
When Pinkie is looking for Pound Cake about halfway through the episode, the house is dark and the only sound other than Pinkie is Pumpkin Cake with the rubber chicken. The window is also open. Suddenly Pinkie hears a whispered "Pinkie..." followed by laughter. It's frightening because she can't see that Pound Cake is flying above her, and up until that moment neither of the twins had ever spoken before. So to her there's an unknown disembodied voice of an intruder in a dark house whispering her name and laughing while she frantically searches for a missing child.
Another thing with Baby Cakes, the Cakes, Earth Ponies, have Unicorn and Pegasus children. in Hearth's Warming Eve, we see that the Pegasi, Earth Ponies, and Unicorns where all hostile towards each other. what happened when, say, a Unicorn gave birth to a Pegasus? they would either be killed or banished, which would be VERY bad for the young pony in question. Or, the whole thing was just a case of mistaken identity with the the little ponies. Which is not a whole lot better.
It's possible the races never, or rarely mixed before then, as they loathed each other. Since Mr. Cake explained that there were unicorns and pegasi in his family tree (or both his and Mrs. Cake's?), thus providing the possibility that the genes needed to be present to allow for it to even happen. Not to mention, any pony not fitting the society may have just been cast out to live in the "proper" society.
If recessive genes can cause Earth pony parents to have a pegasus or unicorn foal, can a pegasus or unicorn have an Earth pony foal? What happens to such a baby born in Cloudsdale? Alternatively, is a non-unicorn going to be mocked and considered a genetic throwback in a unicorn society?
Very doubtful. Perhaps back in the old days before Equestria was founded this could be a problem but in the Sugar Bowl that is modern Equestria this kind of pony "racism" would be absurd. An earth pony would be able to live among unicorns without any problem (remember that Canterlot - the most high class city shown - has plenty of earth ponies in high positions). For an non-pegasus born in Cloudsdale, there may be creative solutions we haven't seen. Or maybe in such an instance the parents would simply move to a different town - a normal sacrifice loving parents are expected to make in such a situation. That or they just simply chuck the babies down to the earth below.
Which brings up a bit of Fridge Brilliance when you think of Hearths Warming Eve. The tribes before Equestria were pretty much, trade with, but don't mingle, or marry anyone from the other tribes.
Imagine what will happen in the twins' minds when Mr. or Mrs. Cake discusses baking a pound or pumpkin cake in front of Pound or Pumpkin Cake. Those kids will probably be given nicknames as a workaround...
It's not as though this episode invented the concept of food names. I've never heard of a real child being traumatized by this kind of thing.
The Cakes deal in baked goods, such as cupcakes and - possibly - carrot cakes. Considering their NAMES are Cup Cake and Carrot Cake, it's probably no big deal. Nothing they can't explain to their own kids. Besides, consider how awkward it would be for half the Apple family...
Rugrats had an episode about this. It'd likely cause more awkward humor mixed with mild fear than trauma. The twins would learn quickly.
The general theory about Scootaloo not being able to fly is that she's simply not old enough yet, which fits with the whole still-growing-up concept of the CMC. Except Pound Cakes is a flight-capable newborn, which suggests flight is something Scootaloo should be able to do by now, but can't. This makes her scooting look more like wheelchair basketball. This also makes her friends teasing her about her flightlessness look a lot more cruel.
Since Sweetie Belle can't use magic while Pumpkin Cake clearly can, it's implied Pound Cake is having a "pegasus magic" spurt, and will lose his flight after about a year and then regain it during, say, tail end of puberty. After all, Fluttershy and Rainbow Dash were admitted to Flight Camp at an age where Fluttershy is clearly having a growth spurt.
Except for the fact that we've now had season 4's "Flight to the Finish", which confirms there's something nebulously "wrong" with Scootaloo's flight abilities.