How old is your daughter and in what grade? If they are passing on messages from their parents then you need to go to the school, ask for a mediated meeting with the other parents without the children being present. Tell the parents very calmly, and nicely, that you understand their position, but that your beliefs are different, and just as they would not appreciate you trying to convert their children, you do not appreciate them trying to convert yours, and that by involving their own children this way, you have put both theirs and your own child in an uncomfortable situation. Explain that your daughter has come home in tears because her basic beliefs and values have been challenged by adults through the children and the pressure is too much for her to bear, and she shouldn't have to do that. By having a mediated meeting, it is put on the table for everyone including the school personnel, they will be aware of it, and they can help assure that she will have a safe environment at school.
Look at the arguments on here concerning religion, adults have no business sending messages to someone elses children through their own. Due to the separation of church and state, t-shirts and other items of clothing promoting a certain religion are really not allowed in schools to prevent just such things from happening. I have to call parents regularly to have them bring their children an appropriate top to wear because of what the one they had said. They don't like it, but then they don't like it when their children are teased and tormented by other children of a different faith or belief. If you are going to wear a bullseye on your back, don't complain when you are shot at. There is a difference between going to church and worshipping, and professing your belief and your faith, and wearing something you know is an inflammatory message to others.
From what you've posted I think your daughter might be fairly young? She wouldn't know better and not realize what she said would be an affront to someone of another religion. Even though she may believe differently, she needs to respect their beliefs just as much as they have to or should, respect hers. As she gets older it will be easier for her to understand that, and be able to take responsibility for a statement such as "I don't believe in God", knowing it will get a mixed response. But still, the parents have no right to send her a message of any sort through their own children.