DS9 still believes, but it could be due to Santa and the Tooth Fairy being pretty persistent. Santa's been known to leave notes and then there were the gifts that were waiting for us at our house when we were two states away..... I'd bet some of this has led to DS writing notes to the Tooth Fairy and well, it was so stinkin' cute that the Tooth Fairy had to write back.
I think that DS *wants* to believe because it's fun and magical. DSs teacher does this really cool thing for St. Patrick's Day involving a leprechaun on the loose and the kids just had a ball looking for this mischievous creature. I remember commenting that I didn't realize so many kids at this age still believe in things like that. I thought DS was a little behind the curve and that he was probably one of the last ones to figure it out. Logically, I think DS has got it, but we make it so much fun that he's not ready to give it up entirely just yet.