Well, has she read Superfudge? (That book thoroughly debunks Santa. We had to turn the tape down in a hurry when listening to it in the car with DS4!)
DD reacted sort of like Dude's DD. She had been pestering me about it for YEARS and I'd said "What do you think?" and blah blah. Finally she INSISTED on the truth, and seemed kind of disappointed. She now has decided it's mre fun to believe, so she goes along with it, also for her brother's sake. She doesn't seem to really care about the tooth fairy anymore, though.
