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?
It's just horrible to see secular people intentionally lying to their kids. It fosters mistrust. Sure, celebrate Christmas, and put the presents in the stocking and whatnot, make it fun. But to lie to your kids about who's doing it seems totally unnecessary and harmful. Same for the tooth fairy. Fortunately for me, my parents didn't lie to me about the tooth fairy. And I appreciate that.