Gosh, what a load of righteous indignation on this thread. Do you get in the faces of parents from other religious traditions about fucking up Christmas for everyone by not making Santa real for their kids, too? Or do you live and let live in that case?
I feel the same way about tricking my kids. (I'm heavily pregnant right now so it's not a huge issue at the moment and I know plans will change in the face of reality!).
My plan is this: during December bring The Night Before Christmas into the reading book rotation and maybe some others. The Snowman might be good, for example. Children "get" that stories aren't real but they are no less exciting to the imagination even with that knowledge.
On the 24th, we can "play" Santa. So, we'll put out the milk/sherry, mince pie/risengrød and carrot etc. Then when the kid wakes up on the 25th, OMG the stuff was eaten and now there are presents in a sock?! CRAZY! Playing Santa is so much fun!
Mall Santas: Oh look that guy is Playing Santa over there, you wanna go Play Santa with him?
But as with all the other games you play with children, I won't insist it's really real. Playing "Mummies and Daddies" isn't less fun because you've not made the dolls set up a joint bank account, for example.
What makes things complicated in my house is the mixed heritage situation. Danish kids have different traditions, the gifts arrive on different days (one a day all December/each Sunday in advent and the big ones on the evening of the 24th but a big guy in a red suit usually drops them off) Nisser (elves) are also a big part of the season and I don't know much about them except they sound like a pain in the arse. Danish Grandma can take care of that, if she wants.
With the potential "spoiling it for the other kids" situation, I guess I'll tell my kid that some people really get into the game and like to go all in. Along with the other children who don't believe at all, goes to show: we're all different. And we don't force people to be just like us, we live and let live.
In summary: I'm going to make it into a game/story time combo and drop all the "omg you won't be getting presents if FC finds out what a shit you are" that seems to go with Advent.