Yes I think let them believe for as long as they want, but I've found they usually find out from school friends anyway. But I do draw the line at this naughty Elf stuff. My ex wife did it with my son and I didn't agree with it; it certainly wasn't entertained when he was with me! We spend many hours telling them that monsters aren't under the bed, and the bogeyman isn't going to kidnap them in their sleep. And then some parents go and pretend that an Elf comes to life at night and moves things! Utterly ridiculous