The purpose of fantasy is to enable children (and adults) to experience what they never could in reality. Every child knows that no adult would send him to the vicinity of a rumbling volcano--which is exactly why they love stories in which things like that happen. Seeing make-believe heroes in danger and seeing them get out of it is one of the ways children learn to be brave and optimistic. If a 4-year-old loves these books, then he's getting something useful out of them. My 8-year-old loves them, and I will happily keep reading them to him as long as he wants!