My kids believed in the Tooth Fairy until she/he kept forgetting to visit for days on end. I told them that lots of kids were losing teeth & the poor Fairy must be overwhelmed. They didn't buy it. But they kept putting their teeth under their pillows and expecting some moola. Silly kids.
(That's just my older three, I have one last young one who hasn't lost any teeth yet-so there's still some magic left.)
Santa-same thing, but he always remembers to visit. (I tell them what my mother told me, "When you stop believing in Santa-he stops bringing the gifts."--funny take on this one-My first Christmas away from home, I walked to the living area expecting & hoping to find that he'd visited. It was a very sad growing up experience for a 21 year old .)