To adapt Superman and Batman for child fanfiction, their costumes can be made more kid - friendly. Superman could have a cape that changes colors depending on his mood (a fun, magical touch). Batman's suit could have little pockets that hold things like candy instead of just batarangs. Their personalities can also be softened a bit. Superman can be a bit clumsy sometimes when he's not using his powers, and Batman can crack jokes now and then to make the story more enjoyable for young readers.
Often it starts with mutual respect. They recognize each other's abilities and dedication to justice. For instance, Superman respects Batman's ability to fight crime without powers, and Batman respects Superman's selfless use of his powers.
Lois Lane is often a popular character. In fanfiction, she might play a crucial role in mediating between Batman and Superman or getting involved in their adventures in a more hands - on way, using her investigative journalism skills.
Alfred Pennyworth is quite popular too. He can offer his wisdom and support to Batman, and his interactions with Superman can be both humorous and heartwarming. He's like the glue that holds things together in the Batcave, and his presence can bring a sense of home to the story.
One interesting storyline could be a young Superman and Batman teaming up for the first time in a kid - friendly adventure. Maybe they are trying to stop a group of small - time villains who are stealing toys from all the kids in Gotham. They use their unique powers and detective skills respectively to track down the bad guys and return the toys.
Lex Luthor is often an interesting character in such fanfictions. He might be the one pulling the strings behind the scenes, fueling Batman's distrust of Superman. Another could be Alfred. His wisdom and influence on Batman can add depth to the story. He might try to talk Batman out of fighting Superman or help him in a more covert way. And then there's Wonder Woman. She could be caught in the middle, trying to mediate between the two or having her own motives for getting involved in their conflict.
Catwoman can also be a popular character. She could be in a love - triangle situation with Batman, and her thieving ways might put her at odds with the female Superman who stands for truth and justice. There could be some really interesting interactions between the three of them, like Catwoman trying to steal a valuable item that the female Superman is protecting, while Batman is caught in the middle, not sure who to side with at first.
Wonder Woman could also be involved. She might be caught in the middle, trying to stop the fight at first. But if she realizes that Superman is being a threat for some reason, she could side with Batman, or at least not interfere in Batman's attempt to defeat Superman.
When Batman saves Superman, it could make him more confident in his own abilities outside of his normal Gotham - based operations. He realizes that his skills as a detective, strategist, and fighter are not just useful in his own city but can be applied on a much larger scale. This could lead to him being more involved in Justice League - related matters and having a more positive outlook on working with super - powered individuals in general. He might also become more empathetic as he gets to understand Superman's vulnerabilities better.
One way is to have a new character arrive in Metropolis or Gotham with unknown powers and intentions. This stranger could start causing chaos, and Batman and Superman would have to team up to deal with it.
One possible reason could be mind control. Maybe some villain has taken over Superman's mind and made him do it. Another reason might be a fundamental difference in their ideologies that has escalated to the point of no return.