Well, yeah... Sam's a thinker. So he's bound to find creative ways to think himself into the answer he wants. He's not "ready" to give up on Santa, so he's going to find a solution that reconciles what he saw (and what he knows deep down) and what he wants to believe. (Sam's also optimistic in that way, since he's the one who still believes in God and angels and so on even in the face of an overwhelming skew toward "evil.")

As for your tot, well, babies know. It's hard to trick an infant, because Mom and Dad feel different, smell different, have that different pheromone. They're not old enough yet to be tricked by visual stimuli / input, which is the easiest to deceive.