Ithink it was
http://en.wikipedia.org/wiki/Lloyd_Alexander, the author of The Chronicles of Prydain, who said that just because a book is meant to be readable by children, doesn't mean it has to be stupid.
said chronicles are btw. a wonderful entry point for high fantasy for the younger ones, and do nonintrusively teach a good pack of values without being sermon-ish. IMO
P.S.: Fantasy is often monomyth variant threads - I wouldn't stick too many age labels on such. Monomyth stories are as old as stories can be...