Writing a funny kid's story is not rocket science. Let's discuss this way.
Point 01: Think about yourself and build a story from your own experience
Point 02: Funny kid's TV programs are good benchmarks
Spongebob square pants. These cartoon programs were very popular among kids in 90's. Why they were so popular funny stories among kids? Answer for this will give you a good hint or beginning to your story. Especially these stories are eye-catching and totally capture the attention of kids because of the characters, these characters are not usual. So children who watch cartoons find specialty out of other television programs. Therefore, it may be useful for you if you include a special role character.