You don't have to worry about informing your kids. There will always be another kid (whose parents are like the author of this thread) that will spill the beans.
Just wait til they find out about "sex" from a third grader. That's always a day to look forward to! :)
I've already had "the talk" ... not about Santa, but about the other "s" word ... with my 11-year-old (who says he believes in Santa but I think really doesn't, he just doesn't want to spoil it for little brother). I don't think it registered. :)