It seems exactly like you’re NOT letting them enjoy the story, just because you didn’t.
This seems like an easy way to rob their childhood of fun memories, interest (watch how much more they care about Christmas movies when they think it’s real),curb creativity, and produce a child who’s overly skeptical.
Kid will also probably ruin Christmas for other children/parents, garnering a reputation as a know-it-all and possibly alienating themselves socially.