As the kids got older I would tell them that Santa would prefer it if we left the whisky bottle out for him instead of milk. :) By then they knew the score but we still tried to keep the kid aspect of Christmas alive. Now they're all grown up but they still come home for Christmas.
One year when our son was at an age where he knew who Santa really was but our daughter did not, my husband borrowed the Santa suit from the school. With our son knowing what we were doing, we set it up so that she saw Santa that year. Christmas eve they were upstairs in bed and I was saying goodnight when I pretended to hear a noise. We all went to the top of the stair landing where we could look down on the living room, the tree and the fireplace where their stockings were and there was my husband, in the suit and all padded up leaving gifts at the tree. Her reaction was completely comical. Her heart was beating so fast and she nearly had a panic attack because Santa Claus! :) If I recall correctly, she slept in our bed that night. I wish I had it on video. :)