they claim to believe in Santa as a way to keep the older ones from spoiling it.
One year my oldest son asked for a specific Star Wars LEGO set that had not been made for a few years. Santa included a note to him why the box was opened and it was a used set "Sorry, my elves don't make this one anymore, but I did manage to get you one. Merry Christmas" - he definitely knew, but played along.
My kids are the oldest of their group of cousins, so they are good at playing along with their younger cousins.
Their favorite thing to have up on the TV during the day is the Santa Tracker showing where Santa is going and who he's been giving presents to.