Children's Non-Fiction: To Be Complete Or Not Complete?

I see that a lot of literary agents require proposals for adult non-fiction and fulls for children's. First question: what are children? I know I know, those little people with big heads that run around yelling things. But in the non-fiction world, there doesn't seem to be juvenile, MG, YA, etc.

I just got an idea for what I think would be a very marketable project for kids in the 7-14 age range and their families. I'd call it 'pop culture' reference, and I think it would be a lot of work (not to mention expensive) so I don't want to write it unless there is, indeed, a market. But I don't want to counter agencies' submission guidelines by sending proposals before I've finished, even started, the book.

What should I do?

Thanks for any help

Well, this is a 'family friendly' guide, so it would only be the older readers in that age range who would be using it themselves. Still not sure how to market it, though. Or maybe I should split it into one for older kids and one for younger.