I'm of the opinion that all kids books should be heavy on metaphor and allegory and all of the other important and joyful aspects of language. Restricting children to books that have been "written at their level" or "made easy to read" is doing them a disservice. It's no wonder so many people grow up to be seldom readers if their first exposure to reading is saccharin pablum that presents no challenge, no meaning, no mystery, and no danger. A proper children's book should be one which an adult can also read and enjoy if they give way to that childlike wonder that still lives within them.