I've posted this over on Hitched trying to gain a wide an audience as possible:

There is a very long back story to this but I want to keep it as brief as possible.

Ex and I had a dog together. The dog became threatening towards the children. Ex refused to have the dog put down, even though this was what the vet had suggested. The house was separated so that the dog was kept apart from the children at all times. We were in the process of getting the dog rehomed. Ex left and took dog with him. He said he would keep the dog away from the children when they visited. My son has told me that he has been allowed to stroke the dog recently and has gone on walks with Ex and the dog. I am very worried for the children's safety around the dog as Ex is blase about the dog.

Is there anything I can do?