I'm around kids a lot, so maybe that helps.
I have a million ideas in my head. It's just a matter of picking the right ones and disciplining them into a story.
For non-fiction, I find that one topic can have so many angles for so many ages, that I just have to sit down and tackle it.
For fiction, I have to "float" and idea for awhile. Either I noodle around with it in my head or on paper. Little bits and pieces of it jump out here and there. When it gets stale, I put it aside and work on something else. It'll pop up again - or not. If not, then I let it sit or delete it.
Read a bunch of kids' books and watch some TV. There are so many creative (and so many lame) ideas out there, it just may trigger something for you.
If it doesn't come to you, then don't write for kids. I know I would have a heck of a time writing a murder mystery. So, I don't.