When Mrs MCTD's older brother figured out the Santa story, he was told that if he didn't believe Santa wouldn't bring him anything (trying to get him to play along for his little sister). he was apparently horrified when he woke up Xmas morning and had no presents!
Turned out Santa had left his in the kitchen by mistake....
My eldest sussed it at about 8 years old. His sister seemed to have it sussed last year at 6, and went shopping for her Christmas and birthday presents with her uncle last weekend. She then got upset cos Santa hadn't taken her letter from the mantelpiece....?
ime, grandparents give the game away. "If there is something you'd like for Christmas let us know and we'll see if we can get it for you..."