If you're familiar with other Unicef ads, you probably won't be surprised to hear that they seem to have quite a thing for this trope. Among other things, they had a little girl play with her dog and step on a landmine (all done in what appears to be crayon drawings), followed by a picture of several heavily bandaged, realistically drawn children. Or the other one where storks drop several babies over a city, they slowly land with parachutes, smiling while looking down on the city or up at the sky with those big eyes, then a squadron of evil looking airplanes show up, carpet bombing the entire city, using bombs with psychotic grins painted on them. The ad ends with the babies landing on the smoking ruins of the city, looking frightened and hopeless. You can tell something like this leaves a big impact if someone can recall it after 12 years with great detail, after seeing it exactly once.
Don LaFontaine: "Something has gone wrong in the happy-go-lucky world of Nintendo."
An infamous animated promo for a 1983 special report entitled “Green Street Reds” for the KGO (San Francisco ABC affiliate) local evening news showed a Soviet SAM shooting Santa Claus and his sleigh out of the sky. This resulted in many angry calls from parents with inconsolable children and the offending clip was quickly pulled. You can watch the clip here.
This ad for 13eme rue, a French action and thriller network. The statement translates to "This is how we'll do a children's show if we have to do one".
Excel Saga has the cute little Puchuus, who are actually violent and imperialistic. When they get injured or killed they shift from cute to plain ol' hideous, resembling Golgo13. There is also a sudden shift from their tiny, adorable cries of "Puuchuu!" to overly dramatic broken English. "Ow that hurt! You make-a me bleeeeeed!". The protagonists seem to enjoy blowing them up and kicking them around.
Higurashi's story is a time loop wherein several characters always start out as schoolyard best friends in a sleepy little rural Japanese town called Hinamizawa. Each cycle of the loop starts out sickeningly cutesy - playground hijinks, characters making nonsense noises ("nipaa") and other characters gushing over how adorable it is, and typical melodramatization of the mundane. Baseball and hide-and-seek, in particular, are seen as Serious Business with the former regarded as an epic battle and the latter subjected to a child's idea of in-depth military strategy. An episode or two into the cycle, though, characters begin to die in horrifically violent, painful and gruesome ways and one character - Rika - is conscious of the cycle of reincarnation and the futility of fighting it. If you've seen the South Park Mysterion/"Coon And Friends"/Cthulhu episodes, you get the idea. She's Kenny. Only rather than being played for dark laughs, it's played just plain dark. Then, when everyone you care about has been reduced to ashes, wet meat, or a comatose vegetable, the plot reset button gets hit and it's time to watch them suffer all over again.
Hibiki No Mahou It features extremely cute character designs, some of them very Moe. But the subject matter and overall tone of the series is dark, considering it's a Deconstruction of the magical school genre.
My Little Pony Friendship Is Magic IDW: We get a chilling vision of what could have happened to Equestria in the form of what the Changelings do to a peaceful land of cute kitty people they just so happened to land in after "A Canterlot Wedding".
In Oz The Great And Powerful there's Oz as a whole, and the land of Teacups and porcelain people in particular. Though there are dangers in Oz most of it highly colorful, whimsical and safe, especially those parts under the good witch Glinda's protection. The "apocalypse" comes in from the wicked witch besieging it with flying monkeys; the land of Teacups is shattered and it's denizens massacred. Main character China Girl is actually introduced after having had both of her legs broken to bits. The rest of the movie could be seen as an attempt to undo this trope and save Oz.
— I'll rid this world of Wishes and fill it with Weapons.
One of the most disturbing moments in Super Paper Mario is when you arrive at Sammer's Kingdom just before the world is wiped out. You try to retrieve the MacGuffin in order to stop it... but you can't make it in time and barely escape. Then, when you go back in... it's gone. All of it. Gone.
The sequel turns things into an out-and-out Crapsaccharine World, with one level having you going through what is in all intents and purposes a torture chamber on your way to off the Big Bad.
The little known shareware game Blip & Blop: Balls of Steel by defunct software house Loaded Studio is all about this: the titular balls start by killing a village of Smurfs (which, incidentally, killed the "great scientist" Gargamel), then destroy (very) thinly-veiled expies of Care Bears, Snorks, Lemmings and various other videogame and cartoon characters. Which are almost all equally evil and murderous.
The end of Doshin The Giant: The final monument is the Tower of Babel, which blocks out the sun. Since Doshin is an embodiments of the sun, that's bad news. Barudo Island begins to collapse, and the villagers flee in terror while Sudoru ponders the nature of life and death. Doshin holds up the tower to let them escape, but since we've never seen any other land mass, we never see anyone actually get to safety, and it's possible for Doshin to walk off the edge of the world, the only conclusion is that everyone drowns. Finally, with the island sunk beneath the waves, Doshin collapses and becomes a new island.
The Maw: Your world is a brightly colored land full of cutesy, silly critters. You're going to eat them all.
The series is like this; a demon trying to give everyone horrible and painful nightmares, the Mirror World imploding on itself, a once-cute-then-suddenly-fanged-beast trying to take over Dreamland by destroying Dreamland... All the while, Kirby is eating everything he comes across. Give or take the apparent thousands of years Kirby could live...
Diablo III has Whimsyshire, a bright and happy land filled with teddy bears and unicorns that are all trying to kill you.
The whole idea behind Naughty Bear is playing the role of a serial killer amongst a group of teddy bears.
Happens to the Dimension of Lame in Sluggy Freelancewhen the demons from the Dimension of Pain finally invade.
An almost literal example with SCP-871. It is a cake that must be eaten. If it is damaged but not eaten, it will be replaced. If it is not eaten within about 24 hours, a second one will appear. If those two aren't eaten, then four will appear. Smart people have done the math and realized that this could wipe out life on earth in approximately 80 days from a single sample left unchecked. There are, at present, 237 such cakes.
If Balloons Could Talk by Neil Cicierega poses the titular question. The first few seconds of the hypothetical world involves everyone happily skipping around with their talking balloons, and then...
There is a story called Day of the Barney in which the purple dinosaur commands his young viewers to kill all the adults. He then takes over using his armies of hooded, mutant "Beloved Ones" and enforces a children-only world by having a ceremony occur whenever a child turns 13-years-old: the boys are decapitated, and the girls are brainwashed, locked into rape camps, and forced to give birth to more half-human "Beloved Ones." The memorable thing about the story? It was played straight (and was surprisingly well-written). In the sequel story, it's revealed Barney is actually an Elder God.
This legend from Hitherby Dragons. And it's certainly not the only one; many, many other stories having coming apocalypses and slaughters happening to a variety of Sugar Bowl worlds. The entire "Unclean Legacy" story is a bleak landscape that occurs after Gargamel has captured the Smurfs, and one stretch of the story of Ink Catherly is in a corrupted, putrefying version of Candyland.
There's an episode of The Fairly Oddparents involving "the Giggle Pies", which are adorable, cuddly, rhyming puffballs of cuteness. They actually are evil, though, and plan to take over the world by subjugating the populace to their cuteness. They are said to taste like manure, which is how the Yugopotamians beat them: Yugopotamians find manure delicious.
Wizards. An evil wizard unleashes his hellish forces on an ugly little hippy-dippy land populated by cutesy elves and fairies. Then there's the big battle scene in which little Rice Krispies mascot look-a-likes get mercilessly slaughtered.
Watership Down. Played straight, then inverted when you find out what the rabbits are capable of doing to each other.
Fiver: The field... the field... it's covered with blood!
Hazel: Blood? Don't be ridiculous... Why don't you go fetch me a cowslip? There's some fine grass over here.
Rainbow Brite: Inverted in The Beginning of Rainbow Land when you find out that Rainbow Land was originally a lifeless wasteland.
People in the Candy Kingdom on Adventure Time explode when they're scared. So Princess Bubblegum has to resurrect the dead. This results in zombies. So she has a huge slumber party with everyone, with Finn guarding the castle, and he can't tell anyone about the zombies. It's actually a subversion.
The Cute King and his army of Cuties accidentally do this to themselves. They always fall apart or explode if they work themselves too much.
In "Magic Duel," another, Ponyville-localized instance of this trope is enacted by theGreatandPowerfulTrixie, who has acquired a relic called the Alicorn Amulet, which enhances her magic to godlike levels. She quickly defeats Twilight in a Wizard Duel, banishes her from town, and converts Ponyville into her own private fascist state. The reason she takes her quest for revenge so far is because, unbeknownst to her, the Amulet also an Artifact of Doom that is corrupting her the more she keeps using it.