I go with the tried and true idea that nobody believes in all gods and i'm just plus one. Kids seems to understand that they don't believe in the norse egyptian or roman gods and so they can consider its largely a matter of setting what youd end up believing. I also try to be gentle and respectful as I don't have a dog in the race.

I'm betting it's a phase. "Talked their parents into taking them to church?". Really?

Sounds like a school thing. All the popular kids go to church so we need to go. Someone brain washed them if it wasn't their parents.

I think six is far too young to try to explain your position. Though you could simply say I don't believe in god(s). That's pretty darn simple.

But a six yo may still believe in Santa so you're discussing this at the risk of their magical thinking.

Do they ask you? If so, give them an honest answer of your perspective. If not then why bring it up? They are nieces and not your responsibility.

Keep it simple: 'I don't believe in God'. Is there anymore 'xplaining' to do? When they're older (14, 15, 16) maybe they'll want to hear about your journey. Hope this helps. Kids matter.