I always think that is the hardest thing about children's books - particularly any aimed at 8+
You look at the likes of Harry Potter, Artemis Fowl and such against, the Lemony Snickett books, and then against David Walliams books, and then add in Roald Dahl.... there is such a vast difference in style, tone and difficulty (not to mention length), that I think it's pretty impossible to pin down any real guidelines other than toning down the violence and any mild sweary words. And even then, if you look at movies that carry a 12A rating they can contain one instance of f*** in them now and are violent as hell, so who knows.
I know my daughter is just turned 5, and she can read as well as her cousin who just turned 7, so even giving a book to a child to test doesn't always work. Hardest audience in the world to cater for - especially when all they want to do is watch an annoying badly drawn walking bacon sandwich and its equally irritating brother.
