A discussion about the belief in Santa Clause is meaningless without the understanding about the meaning of Christmas.
Obviously, Santa Clause, as the old bearded gentleman in the red suit with flying reindeer is a synthesised character. However, it has become the very public face of Christmas for a secular population who are unwilling/unable to deal with the real significance of the season. For those who celebrate the occasion, you will hardly hear Santa being mentioned during a service or a Christmas gathering. OTOH, the other "person", i.e. the reason for Christmas, gets mentioned a lot.
Just because Santa (as in the commercialised TV friendly version) does not exist, does not mean that the reason for Christmas is false. IOW, I will neither encourage nor discourage any belief in this "commercial" Santa, as it merely serves as a secular symbol for the occasion, but rather, will emphasis and teach my children about the meaning of Christmas.