A six-yr-old neighbor (whose family celebrated a holiday other than Christmas) told my 6-yr-old DS that Santa doesn't exist. When DS confronted us about it I told him that if he didn't believe in Santa that he wouldn't get any presents. He's been a "believer" ever since.
My dear dad used to tell us "the minute you don't believe in him, he doesn't believe in you" So my bro and I kept the faith (full well knowing the truth) so I used that with my kids and we never had any drama about it.