Very tough one, no?
I really really believe that you need to let your daughter see her grandparents... I am sorry to disagree with everyone, but it is her right to do so.
Having said that, you need to have supervised visits, and that's going to be hard. You may have to (gasp) have them over to your house. Could they visit while you are working in the kitchen?
Also, they should not visit unless you have them sign a document insisting that they do not talk about their religion on ANY level with your daughter, otherwise they will lose all privleges of visiting with her. Having said that, YOU NEED TO HAVE IT SUPERVISED, NO TWO WAYS ABOUT IT. They will tell her what kind of pagan you are, and how horrible you are.... you must be there with a listening ear.
If you don't let your daughter see her grandparents, then you are going to make her hate you and the religion will seem a way to rebel against you... that is something that you don't want!
Please, think about your daughters feelings, your own situation, and make the decision based on that. I can't see it from both angles... the only thing that I can say is you MUST, ABSOLUTLEY MUST (sorry for the bold - but it's important! ) ENSURE THAT VISITATION IS SUPERVISED, AND ONLY LIKE 1/2 HOUR AT A TIME.
That stupid religion. It breaks everyone aqart, and it's disgusting.