I appreciated our principal at the end of last year. Some of the bigger kids were telling the little kids that Santa wasn't real. The principal told them that if the little kids choose to believe in Santa, that's their prerogative and the big kids shouldn't be telling them otherwise. The same goes for God and the same goes for the Aboriginal belief system (our school is largely Aboriginal). No one is going to believe exactly the same things, even within the same groups. Everyone is entitled to their own beliefs. No matter how truthful or not they seem to someone else. Our school is pretty small so it was easy for the principal to get the message out in one fell swoop.
Regarding the Santa thing specifically, my children (still small) all believe but it's not the be all and end all of Christmas for us. I have seen floating around on Facebook, a letter addressed to a child who had questioned Santa. The parents explained that Santa was a "feeling" not an actual man. The feeling of Santa lives through parents at Christmas time etc. I liked this. I think it is what I will employ when my children start questioning it.
As for the God issue. My eldest knows that his Mummy and Daddy don't believe in God but that he can believe whatever he sees to be true. He says he does believe in God. We are okay with that and we will encourage him to seek any answers he needs. He isn't attached to any religion, I don't think you have to have a religion to believe in God, I find beliefs and religions don't have to be mutually exclusive, but if later on he wants to be acknowledged as part of a religion, that's his choice. Same goes for my other children of course.