Well it depends. Were both parents atheists or religious? Given the ubiquitousness of religion one would hazard to guess most atheists had religious parents. I have no stats to back that up though but there'd be some truth in it.
Personally my mum is devout, my dad less so. I'm grateful for that because it gave me room to question my beliefs. And they were fully instilled beliefs. I was a full on 7th dan catholic as a kid. Baptism, confession, communion, church every week, prayed at night before bed, the whole shebang. (Hail Mary's scared me because of the 'if I die before I wake' line.)
I never indoctrinated our kids though, just kept asking them what do they think when they asked questions around god and religion. Probing them and letting them work it out themselves is something to see.
It's amazing how smart and perceptive kids can be if you let them be. I know you want to frame atheism as a belief system, it's not.
Atheism is a lack of belief in a god(s), not a set of doctrines, scriptures, or shared practices.
You don't, well nobody here does as well, believe in Thor, Woden, Venus, Gnesaha. You reject all those gods.
As Ricky Gervais said, everyone is an atheist to all the other gods except theirs. I've just gone one god further.