I'm working on a big novel for children - sort of 9-12 age range. It's very difficult to pitch it just right, and I find myself having to rewrite big chunks because I've just gone into way too much detail - you've got to keep a child interested, and it's easy to lose them if your story gets a little slow.
I read it to my 8-year-old when he was 7 - he thought it was boring! That meant a big rewrite!
They're a difficult audience to please, and don't be under any illusion that they're easier to write for than adults - they're not, they're very discerning.