I don’t think it matters. I’ve always thought it was really stupid. When I was a kid I didn’t believe it, even as a small child. Then later I felt really guilty about that and decided to pretend that I believed it to make people happy. Then I got old enough to feel embarrassed by being too old to pretend to believe in Santa anymore but felt an immense crisis with the desire to continue to keep my mom from rejecting me since I wasn’t child like anymore so I started to give presents back to my mom as “santa” too.
After a while all our presents were “from santa” and then…
I moved out.
I don’t think I’ll have children so it’s kind of a moot point. I’d probably just forgo the whole thing and if the kid picks up the santa thing passively from other kids just ask it if it believes that. If so, hey… write a letter. Maybe he’ll answer.