The advise given already is helpful. In short, I would say the best way to handle this is to be very direct with their mother as to why they require one of them be present. Try to keep it civil.
Let her know that they will not allow her to indoctrinate their child with watchtower teachings, and they have a right in raising their child to make that request. If she will not abide by it, then she isn't welcome. They don't want that, hate it has come to that, but there it is. She is NOT going to be able to get unsupervised visitation. they do NOT need a lawyer for this. I would have them call her directly and speak to her kindly.
If she insists it is her right to indoctrinate them and see the child unsupervised, they should make a note of her exact language and let her know if she pursues this, then she will be ending the relationship. All she has to do, is stop talking shop. If she refuses, cut her off.