I personally couldn't believe in it because I didn't even when I was younger. I can't remember a time when I ever believed Santa or the Easter Bunny was real. It's also hard for me to understand what exactly makes beleiving in things that don't exist as childish. Isn't the only reason children believe in something like that to begin with because it's forced on them to believe in it? They are lied to and told something is real when it isn't. You could replace the easter bunny or santa with practically anything.
My point is, to me the belief isn't inherently childish. You can even get an adult to believe in things like that if they don't know any better or are just gullable.
I think part of the reason I feel that way I do is because, I don't regress, I am just always in a state of regression in my personality. So to me, it doesn't make sense to believe in something I myself wouldn't beleive. I am already childish by nature, I don't need to pretend, if that makes sense.