Children will happily and convincingly engage in the lovely pretend game about the benign old guy with the reindeer, without necessarily thinking he’s real. That sort of play is one of the great joys as well as benefits of childhood. But they may also end up thinking that Santa really exists with a sufficiently straight-faced adult armed with disappearing milk and cookies. That belief won’t do them any harm either, after all most adult Americans believe in the supernatural.
Psychologist Alison Gopnik in a discussion about children and Santa at the Room for Debate Blog