When I was 7. It was late one night and I was walking around (when I should have been asleep). I noticed my mother finishing wrapping a present and she asked me to place it under the Christmas tree. I think it just slipped her mind in the moment and she didn’t realize what she had done. I didn’t say anything, but I knew from then the presents from Santa were from my parents. I wasn’t sad, but instead felt like I was just let in on a big secret.
Maybe it slipped her mind. Or maybe she was being smart about the moment - including you in the moment rather than making it feel like something's being hidden from you. Just a thought.