"My parents won't buy me a dog." "How did you ask them?" "I asked, 'Mum and Dad, can I have a dog?'" "That's the wrong way. You have to ask, 'Mum and Dad, can I have a baby brother?"
Siblings. Lots of us have 'em. Lots of us don't. The latter situation is often quite common in fiction, leading to a lot of only children running around.
But some fictional only kids (especially younger ones) are not happy with the situation. Maybe they're lonely because they have no one to play with. Maybe they're frustrated because they have no one to push around. Maybe they just see a lot of other kids with siblings in real life and are just jealous. Whatever way, they know what they really want: A younger sibling. And they aren't afraid to ask their parents for one.
Their parents are generally less thrilled about this, because even if they also want another child, they're not keen on telling their kid where said baby is going to come from.
Frequently, kids will express a desire for a sibling the same gender as them: Girls generally want little sisters, and boys typically want little brothers. But the opposite situation isn't unknown: Sometimes boys want little sisters in an attempt to purposefully invoke Big Brother Instinct. Similarly, sometimes tomboyish girls will want brothers... and not-so-tomboyish ones want living dolls.
In works with a fantastic bent, this may be a cue for a younger sibling to suddenly come into the picture through supernatural or superscience-y means. In less-fantastic works, it's often up in the air whether the kid gets their sibling or not. Nor is there any guarantee that, should they get their wish, they and their new sibling will get along the way the older one dreamed.
Compare I Want Grandkids (for when it's the parents' parents doing the pushing). This is also Truth in Television, which is probably why it shows up often enough.
Chocotto Sister is about a boy who always wanted to be an older brother, but who never got a younger sibling... until one day, in his college years, Santa Claus gives him a little sister for Christmas.
In Nanoha The Movie First, it is revealed that Alicia Testarossa always wanted a little sister (which is exactly what Fate is to her), as Presea remembers only while plummeting to her death.
In Baccano!, Isaac and Miria receive a letter from Ennis that mentions her sadness over her "brothers created before her, who [she] never got to meet." They assume she's asking them for a baby brother. And so they bring home Czeslaw.
In the film The Hunt for Red October, Jack Ryan tells his CIA boss that his daughter ("a very precocious five") asked him and his wife to "buy her a baby brother" and settled for them buying one for Stanley (her teddy bear) instead. At the close of the film, Ryan is shown sleeping on his flight home with a teddy bear in the next seat.
At the end of the remake of Miracle on 34th Street, Susan remarks to her mother that her Christmas wishes were a house (instead of their trendy apartment), a dad, and a baby brother. Since her mother marries her longtime boyfriend on Christmas Eve, and Christmas morning finds them moving into Susan's dream home, it's heavily implied that the mother is pregnant.
In The Dresden Files short story It's My Birthday Too, Harry reveals that he wished for a brother when he was a child in the orphanage after his father's death. Ironically (and heartwarmingly), the person he's telling this story to is his long-lost and recently found older brother Thomas.
One adventure of The Baby-Sitters Club has Mary Anne and Dawn wanting their parents to have a baby so they'll be able to baby-sit it. They come to their senses by the end of the book.
The seventh edition of annual Dutch children's choir project Kinderen voor Kinderen included a song called "Ik ben toch zeker Sinterklaas niet" (roughly, "Do I look like Santa Claus?"). The lyrics kick off with little Sascha's wish list: a baby brother, and a pretty red bike, and a giant teddy bear, in that order.
Tomorrow, the World! by James Gow and Arnaud d'Usseau:
Michael: Pat worships you. She comes home from school every day telling me you're the only teacher who has any sense. What's more, she's asked me to get married. To almost anybody! She wants me to have more children. Three of them, to quote precisely. Leona: Mike! Michael: As soon as we're married, I'm sure she'll take it up with you.
In Fire Emblem Awakening, if Yarne is paired up with his father and steps on an event tile, the latter will ask if the former wants something. Yarne asks for "lots of bunny-eared brothers and sisters". This is completely justified, as Yarne's greatest fear (having come from a Bad Future where his mother died) is the extinction of his kind.
In Drowtales, it's considered good luck to have twins or a similar - age sibling. As a result, the Sharen clan has the practice of a "protector twin" (adopting - or buying - a child, usually from a servant family) to give their own children a chance to have a same - age sibling. Unfortunately, this often results in sibling rivalry, ending with the death of the protector twin at the hands of the former child, or (less often) vice versa.
The Oh Yeah! Cartoons short Zoey's Zoo involves a little girl who keeps tons of animals in her own personal "zoo," because her parents (or at least her mother, whom we only ever see) never caved in to her desire for a little brother. Doesn't stop her from asking about it incessantly, though.
Littlefoot in at least one of the The Land Before Time sequels has shown interest in wanting a sibling. It doesn't help that he's one of the only main characters in the entire series (except for Chomper) to be an only child. He DID get a stepbrother when his long-absent father Bron returned in the tenth film, and at Littlefoot's urging, adopted Shorty, a brachiosaurus who had come to view Bron as a father.
Once on The Simpsons Bart, upset about being the Out Numbered Sibling, tries to trick his parents into having another baby. Eventually Marge confronts him about what's going on and explains that she and Homer only wanted 3 kids, and even if they did have another baby, it might be a girl. Bart winds up going to the orphanage and temporarily adopting a brother instead.
Zig-Zagged in the Family Guy episode "Emission Impossible": first is inverted when Stewie hears that Peter and Lois want another kid, and he tries to sabotage any of their attempt at having sex. After meeting one of his possible future siblings inside Peter's testicles (long story), he's eager to have a younger sibling. However, he later made up his mind again.