I'm writing this as someone who has mostly lived in the US and Canada. Personally, I find the whole "lying to children about Christmas" thing just a bit weird (no judgment on those who enjoy this aspect of the holiday). But because it's completely normalized in our culture, this is something many people have to deal with.
Two questions:
What age does this normally happen? I suppose you want the "magic of Christmas" at younger ages, but it gets embarrassing at a certain point.
And how does it normally happen? Let them find out from others through people at school? Tell them explicitly during a "talk"? Let them figure it out on their own?
https://youtu.be/BsR6sIsoWgU
Not sure if it will help, I was looking for another video about Santa specifically. But could not find it. In that video he says that he did not dismiss Santa's tales but also did not engage with it. And at a certain point question the tales and asked the kids how they know that's Santa who is giving them presents. Then the kids looked for ways to find out, and discovered themselves. (If I remember correctly)