I like this. in my family, I figured it out at about 3 or 4, promptly told the 2 year old, and broke the reality to the next two before they could even start to believe there was a real Santa.
instead, Santa was the spirit of Christmas, so any of us could be Santa if we gave presents with no expectation of recognition or a return gift. much more Secret Santa than magical man leaving presents.
this did lead to several years where the youngest would give away all their toys, only to then reclaim them after presents were opened. generosity isn’t an easy concept for the pre-schoolers.