How can I protect my children from a mentally/emotionally abusive grandparent?

My biological mother has been openly "badmouthing" my current husband to my 6, 4, and 3 year old children for the duration of my relationship with him. She and I no longer have a relationship, but she is close to my ex husband and he regularly leaves my children with her for visits. I have confronted her about it and she denied that she had said anything negative about him to the children, but admitted that they may have overheard conversations between her and other adults in her household and agreed to be more careful about what she says in front of them. Since this discussion, she has continued the behavior in front of them and even implied to my daughters (in front of me) that if they went with me for the weekend, they would die. I researched policies on getting a restraining order, but as far as I can tell, nothing that she has done is enough to get a restraining order against her. What can I do to protect my children?