For me, Santa is to the Spirit of Christmas as A Big Bearded Man on a Cloud is to God.
I can honestly tell my children that I believe in Santa. Believing in Santa does not mean I believe that a fat jolly man actually breaks into our house every Christmas Eve, any more than I believe that God actually perches on a cloud somewhere in the sky. Believing in Santa means believing in the Spirit of Christmas, which is a joyous phenomena created by the belief of tens of thousands of people, celebrating the return of light at the darkest time of the year.
Children are literal in their view of the world. That's why children often believe Heaven is "somewhere up there". Adults gradually develop a more nuanced understanding of reality, and can understand that God is incorporeal, that God can exist in our hearts, and that Heaven can be a state of being, rather than a real physical place.
Santa is real. God is real. It's all true.