I completely understand your dilemma (at least most of it). I am an atheist, and I don't want my child to grow up believing in God any more than I want him to grow up believing that the earth is flat. Within my paradigm, these are both things that are simply blatantly untrue, and I would be negligent as a parent if I distorted my child's ability to see truth by trying to get him to believe untrue things.
HOWEVER, at his age, with his imagination going wild, he likes to imagine that all sorts of imaginary things are real, and that is just fine, because imagining that dragons and faeries and Santa Claus are real is not the same as being asked to believe in something like God.
We are never told that dragons control/oversee our lives, or that we need to be thankful to dragons for the good things in us. From my perspective the most damaging things about religion are the teachings (not espoused by all religions, by the way, so I'm not saying this applies in all situations) that the sources of human goodness and human evils are external, rather than coming from within. This cheapens the good things that people do, and partially absolves them of responsibility for the bad things.
I, too, have religious in-laws, and we see them A LOT. I actually rather like them, and in general they are great for my son - they love him a lot, they like to play with him and read to him, and they're really nice to me, too. But they have a very hard time respecting my wishes regarding religion, and whenever we stay with them and my son wakes up before I do on a Sunday morning, his grampa whisks him off to church while I'm still asleep. Likewise grandma used to have him pray before going to sleep on the occasions when he would sleep at their house. I spoke to them each separately about it, and grandma was able to acknowledge that as the parent it was my decision to raise him without religion, but grampa gave me a lot of push back and has kept sneaking my son off to church. They both still sometimes give him books which claim that hope and love and other such things come from God, but it isn't too hard to quietly take those out of circulation when we get home.
Ultimately, I cannot control my in-laws' behavior, and they do my son too much good in other ways for me to consider just not seeing them. All I can do is be consistent in my own home, and keep expressing to my in-laws my concerns, discomfort, and disapproval of the ways in which their actions undermine my parenting and what I have determined to be best for my child in the long run. I console myself with the knowledge that he gets a lot more exposure to me and to his dad than he does to them, and that when he reaches an age where he has enough curiosity to ask, I have studied enough and thought deeply enough about religion and philosophy and history to be able to speak with him very coherently on the subject. But, ultimately, I can't control my son, either, and he will make up his own mind.
I guess I would suggest that you might have good success in expressing your wishes to your in-laws if you recognize the things about them in relation to your daughter that you DO appreciate, and verbally acknowledge those things to them, while at the same time expressing your concern that teaching religion to her at a young age could hurt her later in life, and that your choice as a parent is not to do so, and you hope they can respect that. It sounds to me like YOU may have had an experience growing up that shattered your religious world view, and that it was hurtful to you. Likely part of your motivation is to protect your child from having a similar experience, and it may help to share your story with your in-laws, too, if they're having a hard time understanding where you are coming from.