I am a parent with religious in-laws and non-church-going parents and we decided it was important to be open and honest about how we want to raise our daughter. This applies to almost everything, not just our atheism. They all knew we did not go to church but none were sure of our actual beliefs. We sat with each side of the family and had very blunt discussions about our lack of belief. We basically said "we love having you in our lives and being in a honest, loving relationship. We are atheists and would prefer to not have religion brought into our daughters life until she asks about it. We dont want to say a prayer before dinner at our house but you are free to say a prayer or anything else you would normally do while she is with you or at your house. Please respect us the way we respect you by having this discussion. We love you and dont care about your religious beliefs, we know you feel the same about us." Everything has been perfect since this discussion. The grandparents just enjoy their time with her and dont offer to much advice.