I don't remember ever really believing in Santa Claus. It may have been due to the fact my parents never made an effort to "decieve" my brother and me. We always understood that presents labeled "From: Santa" meant "extra" presents that my mom bought, because she always bought too many presents for us.
It is an AS trait for Aspie children to need logical reason or explanations to do certain things, especially social rituals. As kids, we don't normally accept something because someone says so or does it themselves - not because we're more logical aor "advanced" as children, but because of that autistic disconnect we have in learning social behavior by intuitively "mirroring" the behaviors of others. We don't learn social behavior the "normal" way, so our young brains have to fall back on what it can do, which is discerning cause and effect. For example, all children learn that if you put really hot food in you mouth, it hurts. Cause and effect - very simple. We all learn that way. Aspie kids will try to apply that way of learning to more complex social behaviors and rituals, and this means we subconsciously start dividing things into "proven" and "unproven" categories. And that from which we can't discern a believable effect-cause relationship of whatever reason, we often discount or distrust.