The concept of teaching a child that Santa Claus comes at Christmas with our gifts seems harmless to many parents. They enjoy the excitement of the child waiting for Santa to come and leaving cookies and milk for him and carrots for the reindeer. They enjoy being secretive about their part in the gift giving. There are also many cute books and movies about Santa that are enjoyable to watch together. So what could be wrong with all of that?
I would suggest several issues that when considered make this not so harmless. First of all, the characteristics of Santa are 1) He is omniscient (all-knowing). He sees you when your sleeping and knows if you've been bad or good. It's a cute song I'll admit. 2) He is omnipotent (all-powerful).He can get any gift for you and some movies indicate even more magical powers to help you in your life. 3) He is omnipresent(is everywhere at once) He can be at every child's home in the whole world on one night and he has been observing all children all of the time all year long. 4)We don't actually pray to Santa but we do make our requests known (like the Bible states) through letters. 5) He is never actually seen. You have to believe in him without seeing him though He has lots of look-a-like helpers you can see. These qualities are all qualities of the Lord God. So have you thought that Santa is actually a substitute for God and therefore God would call him an idol which He makes clear in the Bible He is very much against.
Now suppose you love and trust the Lord God and you want your child to love and trust Him too. You know there will come a day in your child's growth when they will realize that Santa is just a cute fairy tale. They will also realize that you have lied to them their whole life up to now. What does that realization do deep inside them about 1) their belief that they can trust you and your counsel and 2) their belief that they should trust your teaching about God and His existence. Is it any wonder that they don't follow your counsel as teens and have a tendency not to trust God or even scoff at the idea of His existence?
It is my belief that I have developed over 70 years of observing families that there is never a good reason to lie to your child. Christmas is not about Santa or presents. Christmas is about the greatest present ever given to mankind, a Saviour who delivers us from our natural tendency to be selfish and make wrong and damaging choices. It's about a great and Holy God who bowed His great authority and power low enough to love us sacrificially. It's not about GETTING it's about GIVING. It's not about receiving presents. It's about receiving ETERNAL LOVE and LIFE. Santa is a very cheap, tawdry substitute for that!