Personally, I really don't think it does them any harm.. I mean look at all the terrible things our little ones are subjected to on the news every day (eg), a least to some degree, even if we switch off quick!.. and bad language in the street.. and then as they get a little older, films with violence in.. computer games etc. It's very hard to keep children away from all of these things however hard you try. What I am trying to say is that, in comparison, a little religion, even if it is not in keeping with your beliefs (or lack thereof) surely can't be all that much of a problem. I understand questions about the crucifixion etc can be hard to answer (no easier if you a devout Christian I guess!).. but little children, with enquiring minds, want to know all about everything that captures their interest ata 4 year old (or whatever) point of view.. and with a little thought, I find I am able to answer these kind of questions in a way they can understand and that satisfies them (for the moment!) (And if I can, anyone can.. .. I'm not a parenting guru! )
The school curriculum is full of stuff which we might not choose.. or think is important.. for our children to be taught.. I feel that way about algebra, lol! And if you celebrate Christmas, then it's natural that school will teach little ones what it is all about.. which is the birth, and subsequent life of Christ. Can't be avoided really unless you want to stipulate otherwise.. which you obviously don't.. and I don't blame you. I think most of us celebrate Christmas don't we? Unless our particular faith does not..and you are not religious at all.
Your DS will no doubt choose his own path in life, re religion, whichever kind.. or perhaps no religion at all. Yes little ones are succeptible to early influences, but as he grows he will be taught about other faiths too.. probably is being already, I understand this to be on the infant/primary curriculum, and I was a school governor until recently. I wouldn't stress too much about this.
Hope I haven't offended.
SJ x