Very interesting. It wouldn't surprise me that most kids, presented with a Santa in the way you describe, would buy into it, even if they had seen their dad dressed as Santa, and done all that "how could he possibly visit all those houses so quickly? And we don't have a chimney!" business...
Our eleven-year-old tells us that she still believes in Father Christmas, but we are not sure we believe her.
She certainly knows there is no such thing as a tooth fairy, but this never stopped her expecting to find a pound coin under her pillow after losing a tooth.