It's common for children's authors and the creators of kids' shows to make the main characters of a story a girl and a boy. They could either already be friends at the start of the story, or they could meet as the plot unfolds. Sometimes they don't even like each other, but are forced to work together. Sometimes the differences between boys and girls create conflict. Either way, it's very common in fiction. So much so that, like all tropes, it's probably done for a reason, especially since in real life, most kids have primarily same sex friends.
So why is it done so often in fiction?
One reason is to expand the potential audience. While some types of stories appeal primarily to one sex or the other, there are many types of stories, such as horror or adventure, that have appeal to both sexes. To try to rein in kids who might not want to read a story about a member of the opposite sex, a character of their own gender is included. Sometimes a story that appeals primarily to one sex will include a character of the opposite sex to expand an otherwise limited audience.
Another reason is because many authors simply like seeing the way kids react when paired up with the opposite sex. It can be fun to see the interactions of two people who at times see the opposite sex as being like space aliens.
While groups of friends may be a mixture of three boys and a girl or vice versa, or an equal mix of members of both sexes, that isn't this trope, despite often being done for the same reason. This trope only refers to when a single boy and single girl are the main characters of the story. Why? Because that's far less likely (unless they're siblings) than a group of mixed gender friends, and therefore more starkly obvious, more clearly done for the sake of the story.
Note that the relationship must be platonic. Romantic relationships don't count, since those involve a more believable reason for two opposite sex kids to get together. Siblings don't count either, since that's far more common in real life and less likely to feel "forced" for the sake of getting a girl and a boy together.
- Zita The Space Girl has Zita and Joseph, who, not including the aliens, are friends. Particularly before the adventure begins.
- Unlike the book version, Coraline adds Wybie, a boy, who joins Coraline on her adventures. Word of God says this was done so Coraline wouldn't have to narrate to herself the whole time.
- The titular characters of Lilo & Stitch are even more unlikely than usual, given that one's a genetic mutant alien.
- In Dinosaur Island (2014), Lucas is 12, and Kate is 15, but there's no hint of romance between them.
- The Amazing Panda Adventure features Ryan Tyler and Ling, who are united over a panda cub and have very little romance apart from the very end.
- The lead protagonists of Big Fat Liar played by Frankie Muniz and Amanda Bynes. Word of God says this was deliberately invoked in the film as very seldom do you see this trope in live-action.
- Goosebumps, nearly the Trope Namer, has kids of opposite sexes as the main characters all the time. Sometimes it's opposite sex kids who are friends, and sometimes it's lampshaded: "Some people think it's weird for a boy and a girl to be friends." Sometimes a kid meets someone of the opposite sex and they get caught up in something big together, and sometimes the kid even dislikes the opposite sex, but they still get caught up in the adventure together.
- Fourth-Grade Celebrity has Casey, a girl, who has Walter, a boy, as her best friend, while she writes letters to a female pen pal.
- My Teacher Is an Alien groups Susan together with Peter, a nerdy smart boy she feels sorry for, and the two end up going on a mission to prove that their teacher is an alien.
- In Encyclopedia Brown, two of the main characters are "Encyclopedia" Brown and his female bodyguard Sally Kimball.
- America's Most Wanted Fifth Graders. A boy and a girl are best friends, and get caught up in a mystery together.
- C. S. Lewis likes this one, using it in four of the seven The Chronicles Of Narnia books:
- In The Silver Chair, Eustace and Jill escape from their Boarding School of Horrors for a romp through Narnia. Puddleglum, a Narnian native, rounds out the Power Trio.
- The Horse and His Boy features the Rebellious Princess Aravis and the peasant boy Shasta. While the narration states that they marry years later, they're too young and too busy for romance during the story proper.
- The Magician's Nephew stars neighbors Digory and Polly, who decide to search a secret attic passage and accidentally stumble onto the titular magician, who sends them world-hopping for fun and profit.
- The Last Battle brings back Eustace and Jill to witness the final days of Narnia.
- The two lead heroes of The Dreamstone are Rufus and his female best friend Amberley. They start off a straight case in the pilot, though many later episodes downplay them into Hero Antagonists to the all-male Urpneys.
- The two leads of The Seventh Tower are a boy (Tal) and a girl (Milla). They are working together more out of circumstance than anything, and for a fair amount of the series they don't actually like each other that much, and the one time the possibility of romance is brought up, Tal's reaction is that he's never even thought about it. By the end of the story they might charitably be called "friends", although "comrades-in-arms" is probably closer, and there's not a hint of romantic tension between them.
- Deadtime Stories mostly uses large groups of kids of both sexes, which isn't an example of this trope. However, the episode "Revenge of the Goblins" has a white girl and black boy who are both friends, with no other friends being shown or mentioned.
- Ben and Holly's Little Kingdom stars boy elf Ben and girl fairy Holly.