With picture books, the problem is that a lot of kids love to sit next to someone and be read to, and will like almost anything. And some hate it, and will wriggle away for almost anything!
For older kids, I think its even harder to find an honest, articulate one than it is to find an honest, articulate adult. There's nothing like a writers' group, one you've been in a while by preference, for telling you the truth.
I wonder how it would work to put an ad up for kids who like to read and would be willing to be in a focus group? You'd have to pay them, and get permission from their parents -- and God knows where you'd do it -- but it sure would be interesting.