Santa is not a fictional character. Santa Claus means St. Nicholas. St. Nicholas was a real person...a bishop in the Catholic church. History has it that he gifted poor families with dowries for their daughters so the daughters could marry.
In part, Santa Claus carries on this example of giving unearned perhaps undeserved blessings. Children are concrete beings. Santa Claus also, in part, represents our relationship with God. The love, the desire for our happiness, the benevolence...it is all a reflection of God's love for us.
Santa Claus isn't an anti-Christian tradition at all. Sadly, some people are unable to separate capitalism and Santa Claus. I think that's a loss for them.
In my home we have an advent wreath and nightly prayers around it. We have a Jesse Tree, where, during dinner each night in December, we learn about one of Jesus' ancestors and how he/she prefigured the savior. It is possible to instill a belief in Santa from a Christian viewpoint.
Personally, I know that Santa exists. He exists in the friends and coworkers who, knowing that emergency surgery caused me to go 2 months without a paycheck, handed me an envelope saying, "God told me to do this." He exists in a friend who, upon purchasing a new computer, gave my boys her old one so they could play games and do word processing. He exists in everyone who offers unsolicited charity. I believe, and I hope that throughout their lives, my boys will gladly both be Santa and accept the gifts he sends their way!