pizza, I am sure that anyone in the environment could see his deplorable behavior.No child should have to be in the position of defending their mother. That is too much pressure on their young shoulders. Sure, they love you, and I am sure that you feel validated by their defending you---but, they will carry the effects of this into their own adult lives and their own relationships in any number of ways.
No child should be or be allowed to call a parent vile names. Even if the parent deserves it. I hope that you will arrange counseling for the children and yourself to find healthier ways to protect them and give them a better outlet for expression of their own anxiety and stress regarding this family situation.