It's an commonly-held belief that 'the best children's books work for adults too'. Into this category are dumped either the children's books written with one eye on an adult audience (the picture books with rude jokes or the 9-12 year-old books which mention Plato), or the books aimed very firmly at children which are just SO DAMN GOOD that everyone loves them.
Astrid Lingren (of Pippi Longstocking fame) was very against the first sort of books. Why put in a joke that children won't find funny in a book AIMED AT CHILDREN? Why make a reference that your target audience won't get? She believed that children's writing should be aimed directly at the people it's for - the children - and that anything aimed at anyone else should be deleted.
(This doesn't mean she was against clever references, btw. Terry Pratchett includes just as many references in his children's Discworld books as he does in those aimed at adults. They're just references to the Famous Five and Hans Christian Anderson, rather than They Might Be Giants and Aristotle).
I'm on Astrid's side (I think), although I think kids are cleverer than adults give them credit for, and I'm not afraid of making them work a bit when reading my books. I also don't think its true that a good children's book will be loved by adults. Adults have singularly failed to get Jacqueline Wilson, Enid Blyton and Roald Dahl. Does this make them bad children's authors? Or are they better authors because they tap into something that cleverer authors have forgot?