To a child raised with the belief in Santa's uncollapsed quantum state, the song "I Saw Mommy Kissing Santa Claus" is very sad.
One of my coworkers, Jeff, came up with the original statement that spawned this whole comic. Really, though, observing Santa would be the worst thing for all the other kids. Though, I'd assume that the child who observed santa would get then all the presents. Or perhaps none, as a result of naughtiness.
If I ever have kids of my own, they're going to have one hell of a weird childhood. Apart from constant tests of their cognitive capabilities and a little bit of Pavlovian conditioning, they're going to be raised with an interesting and intricate set of lies to explain common childhood beliefs. Ideally, at various stages each, lie could be dispelled by a simple experiment. Instead of being told at the age of eight that the Easter Bunny is not real, they will be able to devise an experiment and disprove their own basis for the Easter Bunny's very existence. Santa will go down last.