I couldn’t answer how many. But speaking only for myself, I was raised terribly Christian. I started to question those teachings at the age of 13 and balked completely at 16. It was all a theological matter to me, too—it wasn’t that I wanted to do ‘forbidden’ things. Those came much later, and forbiddenness wasn’t the issue.
To me atheism is a belief, not a lack of one. I am not without beliefs. I am without belief in something that others call God, and I am also a believer in the nonexistence of such a being, every bit as much as I am a believer in the nonexistence as divine entities of Amon-Ra, Tammuz, Zeus, Jupiter, Thor, and all their brothers and sisters.
I’ve raised my own children as honestly as I know how, which meant not teaching them anything I didn’t believe myself. We treated Santa and the Tooth Fairy as games that are fun to play, but I would never make a single assertion that “Santa is ~~” or “the Tooth Fairy will ~~.” I simply would not lie to them at all. They seem to have managed very nicely without those deceptions.
I also taught them not to mess with other people’s beliefs.