I'm not going to say that I agree it's important that a chill believed in peter pan or any certain fairy tails, but I will definitely say that watching or reading those old fairy tales as a young kid I think it gives then a little more imagination, they might not believe what they read but to a kid anything seems possible. So many kids these days grow up watching either tween Disney shows that seems to expose them to the idea of "drama" at a very young age, or they see these weird cartoons on nick or cartoon network that don't seem to provide anything. They sit around by the time they're 8 and go I'm bored!!
Let them decide if they do or not, why would you be so concerned weather or not they believed in him. if they don't seem interested in fairy tails try ready to them about other things like animals, and whatnot.
Why would you want your kids to believe in that stuff? It's cute but you will spend all of this time convincing them that these fictional characters are real only for them to find out latter that they are not. Plus how would you feel if your child was convinced s/he can fly and tries it.