You're painting with an awfully big brush. Certainly there are times when the children in Narnia do things that would be considered "evil" -- in "Wardrobe" Edmund, of course, betrays his siblings; in "Dawn Treader", Eustace is a selfish prat, and even Lucy succumbs to envy and starts to read the spell in the Magician's book to make her more beautiful than Susan. There are good magicians and bad magicians in the book -- it is the characters actions, not their vocations, that make them good or evil.