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.

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.

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.

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 also wouldn't bring it up unless asked.

I agree with @jlynn37- if it's not broken, don't fix it.

But if they bring it up, I suggest dealing with it gently and with compassion.

Maybe ask if they believe in other things (Fairies, Dragons, Ghosts...) you may get to a point where one of them believes in something, and the other one doesn't, but if not, you can maybe suggest that some people do believe in the things they don't believe in.
At that point, you can say that it's the same thing with God - some people believe and some people don't. You can even (bite your tongue and) say that it doesn't make one person right or wrong, better, nicer, whatever... just different.
Good luck.

Bring it up now and you'll be the monster. Bring it up in their teens amd you'll be the cool aunt.

I was just matter of fact about it when my nieces and nephews were very young. They prayed, I didn't, so it came up early. It has never been an issue except at one point where they worried for my eternal soul. They have since gotten over it.

Unless they bring it up, its a non-issue. Its not like you're bringing around your same sex partner.

Check w/ the parents & see what they say.

If the kids invite you to church, decline politely. If they ask you if you've heard about god, say tes, you know those stories & change the subject. Until they try to pin you down, its a non-issue. When they make the move to pin you down, be honest, forthright, & just tell them you don't believe & if you need to, the why.

I’m looking forward to having such conversations with my 4.5yo daughter, but she’s my daughter. So when the time comes I’ll be delighted to explain it to her.
I wouldn’t get involved as much with my nephews and nieces. I’ll let my sisters bite the bullet when they have to!

Maybe just say something like "Some people believe in God and some don't. I'm one of the many who don't." That gives them somethig to think about, without being judgmental. If they love you and think you are a good person, it helps them understand that a belief in God is not what makes a person good or lovable.

You could explain that God is like Santa but for delusional grown ups. Also I would encourage that you prevent them from being indoctrinated early before any permanent damage is done. Indoctrination into religion should be considered a form or child abuse IMO.

It's a good thing to introduce another side to the coin. It's reality. Being reticent is only encouraging them to be insular and think their way of thinking is widely excepted. Different people believe in different God's, different traditions and some dont. Athiesm isn't dirty.

Your someone who keeps fit

Although their parents may not like this. . .

I would begin with a discussion of what is real (can be demonstrated to be possible) and what is not real or a fairy tale story.

After you establish the difference between make believe and what is possible in the real world I would begin with an explanation of how a fish's digestive system works and if it were possible for an air breathing animal to live for three days inside a fish's mouth or stomach.
then introduce a few bible goodies like Jonah and the wale (big fish) and ask them what type of a story this is. Matthew 12:40

You could also talk about how we make voice sounds (vocal cords) and ask them if snakes and donkeys have vocal cords. Then ask them if a story that has a talking snake (or donkey Numbers 22:28) is a real story or a make believe story

or you could talk about what would happen if the earth were to stop revolving (how much energy it would take what would happen to people on the earth then tell them a story about the sun not moving)
Joshua 10:11–13 Again, ask them if this is a real story or make believe.

If you wish you can go on to the great flood and how a small boat is not capable of supplying 2 of every species and how the oceans would become fresh water killing all salt water fish. . . What would the animals eat after leaving a boat with the entire eccosystem destroyed by floods? How did the penguins get there and back?

The list goes on, Then ask them if a book with these stories is a factual (based on reality) or a fairy tale book (imaginary)

You could conclude that because you believe it is important to believe things that are real and can be demonstrated to be true and you have not found any reason to consider their fairy tale book a reliable source for truth, you do not believe in their story book that tells about a god that we can not demonstrate to be true.

I'd talk to their parents first. Have they asked you?

I think most children are atheists anyway. They're a lot more perceptive than adults on the whole.

A few months ago, I had to explain to my 9 year-old, great-nephew, that not everyone believes in the same god he does, and some people don't believe in god at all.
I used examples of children living in places like India and Japan, and how their gods,
and their religions are different. I also told him that not believing in his god doesn't make anyone "bad".
I haven't gotten to telling him I don't believe in his god, or anyone else's. That's a lesson
that is coming, but hasn't happened yet.
Just tell the truth.

I'd tread carefully. In particular, if they're not curious about your beliefs, I see no particular reason to offer them up.

If they're actually asking, simply be honest. Not everyone believes in the same gods, even people who believe in the same god have very different ideas about that god, and some people don't believe in god at all. You're one of the latter.

Just because a kid is curious enough to ask to go to church is no indication how they'll end up, by the way. My stepdaughter begged at the age of your nieces to go to church and to have Bible stories read to her. As a senior in high school she attended catechism. It was presented as an objective inquiry and she was encouraged to arrive at her own conclusion. She scandalized her catechism class by being the only person to conclude that "god is not real". She has not been in church since.

Honestly in her case I think she is just very social and many of her friends (and her biological father) were at this church, and as she moved on from that and matured, she saw the truth, plus, no longer saw the church as a neceessary hub of social contacts.

The very reason l had no children was l didn't want to get into philosophical discussions with a six year old.

I don't feel it's necessary to explain this sort of personal choice to kids.
You deal with people the same.

Hey, child, that is a person over there.

Someone that believes in people not invisible sky wizards. lol

I handle nearly all matters pertaining to questions asked by children like this:

1. If they don't ask, don't offer information, especially on a sticky subject (Sex, Religion, Politics - you know, all the fun ones).
2. If they ask about something they do, that you don't, like going to church, let them know that you're lucky because you're an adult, and you get to choose, and you chose not to go, even though lots of other people choose to go.
3. Never answer anything more than the minimum for what you were asked. When my son was about 7, he said to me "What is all this I hear about the Birds and the Bees?" I thought, oh, boy, here it comes... when I answered, I just said, "they're talking about sex, Honey", and then I cringed, waiting for the other shoe to drop. His response was "Oh, okay". End of discussion. Of course months and years later, I got more questions, but I dodged a bullet that day.
4. With everything else that kids ask you, like many others below have said, its always best to be honest, considering their level of understanding and their feelings. They know when you are decieving them. Just don't over-explain.

Tell them that just like Santa is a marketing ploy to sell consumer goods and toys in an otherwise dead season, God is a marketing ploy to sell control to those lacking self determination.

Then tell them an Atheist is like Santa, but real, and bringing truth instead of toys.
Encourage them to become Atheists and get all their friends to join them.

Then take their ice cream and go on about your day, knowing you've made a difference in their young little lives and teeny tiny brains.