Teaching your kid about Santa Claus is a bad thing. You may think it is harmless and something cute and everyone is doing it so it must be ok, but perpetuating the myth that a fat man lands on rooftops, sliding down chimneys to deliver gifts to the whole world and ignoring a bright kid's inquisitive nature about how it is physically possible to travel to every inhabited location on the planet in one night or exactly how this jolly man gets access to houses that have no fireplaces, well you're basically just showing your kid that you can lie to him.
It's no different then a guy who bold face lies to his friend that his fly is open then promptly fingers his nose as he checks out your fictitious observation. The victim is going to think twice next time you tell him something that might be in his best interest. It may be funny to you and the victim may appear to laugh, but something dire has just happened.
You think since they were kids they will forget. But you don't understand the power of the subconscious mind. You have irrevocably damaged your child's trust in you for the rest of his life.
It's better to come clean now and instruct the child in the importance of rituals and culture should you continue to observe Christmas in its playful ways.
Oh, and the same thing goes for the Easter Bunny, Harry Hanukkah, Jesus, etc.