"I wonder how my 10 year old has made it to 5th grade without hearing that Santa isn't real. I'm so thankful that (at least in my area) it isn't a priority to crush the childrens dreams of Christmas joy."
That's funny. Your kid knows. It's just playing along with you. Santa is fun. Even when you know he's a "pretend" character. I still sign Santa on some of the gifts I give my wife, and I promise that she knows where they really came from.
But...you go ahead and believe that your 10-year-old still really believes that Santa exists and lives at the North Pole and flies a sled pulled by reindeer.
I still sign my kids presents Santa and they are grown. One kid commented Christmas eve, "since you don't have a chimney, does he come in the sliding doors?", a friend commented "he'd better not, your mom would blow him away with her .38 before he made it inside". He's just fun, let the kids have their fun.