Many children may be raised as atheists in an atheist family, but since most atheists don't make a big deal out of not worshiping a deity, chances are very good they won't be preaching to their children either. I know a number of people who are atheists, raised in a family without religious training, but never were taught about atheism by parents.
I think it always comes down to faith in God.
If you have faith chances are you were taught it when you were a kid, and subject to experience, it remains with you.
I don't believe it likely faith will appear in a vacuum (although I believe it is possible). Faith doesn't grow on trees you know.
So no training = "no faith" = no belief in God.