Mm, I didn't want to do a serious post here but, ah well. . .
Back when I was a child, I wasn't really taught anything at all in terms of beliefs. No Santa, no easter bunny or the like at all. Hence, as opposed to having a primary belief and individuals telling me what's not true, I had the situation of having no belief and individuals telling me what is true.
And now I passionately believe in nothing and yet accept the possibility of everything. And looking back, I think if I was not given some exposure to other possibilities, I would probably be a lot less accepting now.
I believe that if children want to believe in something, they will believe it (even if blindly) no matter what people say. If they believe that Santa exists, he will exist to them. But as 6.25's quote of lunarea's post points out, if someone flat-out tells them that something like Santa that they believe in doesn't exist, it wouldn't be very ethical to do in a way - yet I think it's not because that it would shatter their beliefs, but because it would upset them in a similar fashion to an insult.
The tradition of Santa and the like have been passed on for longer than people can remember. Through all this time, the "system" seemed to work fine, and it's highly unlikely that there haven't been many cases of children being told that certain things don't exist.
I think the bottom line is, children will stop believing in something when they are ready to stop. If they are able to reason, think and understand when they are being told something against their belief and actually accept it, it means they are able to come to a conclusion of their own reasoning, hence conjuring what is ultimately their belief, instead something they are simply told to believe in.