Im not a father-to-be (A lot of people would agree this is a good thing), but recently I had one of my little cousins (who's about 10 or so) come up to me during a family during a family dinner after a wedding, and ask me that question. I think she had been asking a few people over the night, for reasons that only a 10 year old can understand.
I just told her what I think.
"No Jess, I don't believe that god exists, but, when you're older, you'll be able to make that choice for yourself"
Now, im the only one in a big family who will not regularly attend a church, nor do I believe in religion itself. so there was this VERY awkward silence.
To which she replied, "Do I have to be old like grandma?" And thankfully, that broke the tension. (Kids say some pretty funny things sometimes).
I think i made the right choice, I answered what I believe, but also told her that its up to her to decide. And I would probably answer the same way when I do decide to have children.