Atheist parents, a question please?

Your responsibility is to your child not your parents. You have to have tell your parents that their beliefs are not yours and that they are doing themselves harm by "preaching" their beliefs to your child. (Frankly, they should be ashamed of themselves.) Unfortunately it might come down to not allowing them to be alone with your child. You should also let your child know that her grandparents have silly ideas sometimes and they should not be believed when they put down other people. Good luck.