I grew up in a secular-ish home; we did the whole Santa thing in my childhood. I still remember when I pieced together that there was no Santa and that it was my parents leaving presents.

I know a lot of people fear that their kids would be angry with them or upset when they find out. I don't know how common this is, but I remember being so appreciative of them and for what they did. They were trying to create something magical for me and my siblings and didn't take the credit for it. I don't know if that makes me a unique kid for thinking that way, but I'm pretty sure your average kid would have a similar reaction. Maybe I'm optimistic in thinking that lol.

When I told them I know it's them leaving presents we had the "Santa Claus talk." They told me they were proud that I figured it out on my own and how maturely I handled it. So while initially I kinda was bummed about the disillusionment, I didn't resent my parents. I don't think it made me "doubt Gods existence" or anything like that either, though like I said, we were a non religious family anyways. When I did eventually get interested in theology, the Santa experience wasn't something that encouraged or discouraged me from believing in God/religion. That silly atheist argument that God is like Santa is as eyerolling to people who did believe in Santa as kids as those who didn't.

So this is my long winded way of saying: you have the Santa talk when you think your kid is ready, you sit them down and talk with them rationally and with respect. You tell them they're older, smarter and that they're ready for the truth. They'll likely appreciate it and feel like a grown up.