First of all I can't write the story for you but I can help you brain Storm.
1. Choose the character you know the most about If you know Super Hero's,Cartoons etc. use what you know then it won't look like you're randomly throwing things in.
2. Figure out witch part of the brain you want to talk about, the facts you want to put in there, and organize your facts.
3. Put it in little kid language, use small words and not BIG words 5-6 year old's have a good Vocabulary but it is better to use small words.
Ok hope you have fun writing this story!