I like the idea of giving them some money every Sunday to give to the offering. It gets them in to the habit of tithing while they are young. But you also have to teach them the importance of giving out of love once they are old enough to understand why they are doing it
My parents never did the whole tooth fairy, Santa Clause, Easter Bunny thing with us so I'm not really sure lol. They did give us some change when we lost a tooth, but we always knew it was from them. If you don't want to do money, you could use small toys or something.