One idea could be a story about a magical tree house. The tree house can transport kids to different time periods or magical lands. They could meet historical figures or mythical creatures there.
Look at their daily experiences. A story could be about a kid's first day at a new school. All the nervousness, making new friends, and getting lost in the big school building. Add some magic to it, like a friendly ghost that shows the kid around.
Well, in a fanfiction where Peter Parker turns into a kid, perhaps he could end up at his old school. He'd be faced with bullies again, but this time he has the knowledge and experience of being Spider - Man, even if he's in a kid's body. This could lead to some interesting scenarios where he uses his smarts to outwit the bullies without using his powers directly. Or, he could befriend his younger self's friends in a whole new way, seeing them from a different perspective.
Another fanfiction could be set in the past, showing how Medusa and Pit's relationship started. It could delve into their first encounters, how Medusa's jealousy or hatred towards Pit began, and how Pit initially perceived her as an enemy. This backstory could add depth to their characters and their relationship in the overall Kid Icarus lore.
One idea could be that the kid has unique powers. Since Percy has his water - related abilities and Artemis is the goddess of the hunt, the kid might be able to control water in the wilderness, like making streams appear in the middle of a forest to help during a hunt or to create barriers against enemies. Another aspect could be the kid's relationship with the other demigods. They might be seen as an oddity because of their parents' unusual pairing, but also be highly respected for the combined power they inherit.
One idea could be a story where Crona and Kid are forced to team up on a mission. They start off not getting along because of their very different personalities. Kid is all about order and symmetry, while Crona is more timid and has a darker past. But as the mission progresses, they begin to understand each other better and learn to appreciate their differences.
One idea could be that Altair, after being hurt as a kid, is taken in by an assassin. He trains hard to overcome his pain and fear, gradually becoming a skilled assassin. He might have a mentor who helps him channel his anger into his training and missions.
One idea could be that Pit is kidnapped and Hades tries to corrupt him. Maybe Hades offers Pit power in exchange for his loyalty. Pit, being the heroic character, resists but has to find a way to escape while also facing the internal struggle of being tempted by such power.
Another possibility is that the kid is from an alternate dimension where time passes differently. Natsu's kid shows up in the Fairy Tail world and he has to figure out how to be a father. The story could focus on their journey to get to know each other and the kid's adjustment to this new world.
Another interesting fanfiction could be set in a scenario where Kid Gohan has to teach Kid Videl some of his Saiyan powers. This could involve training sessions filled with funny moments as Videl tries to understand and control the newfound abilities. They could also have some friendly competitions to see who is stronger at different stages of her training.