My childs' stepfather is abusing MY children what do I do? I want to help but, she is making it hell on my kids.

Tonight, my childrens' stepfather held a drill to my child's throat and said, "Go ahead do you think I won't do it motherfucker?" This after he threatened to "bust all of your teeth out" and "you're lucky that I don't push you down the stairs". I showed up on my day to get my children, but she would not let me because she suspected that I "smelled like pot". When I went to get them, she wouldn't let them come with me because I am a loser who "smells like pot". Yet, I have never spoken to them in anger, or hit them in anger. Meanwhile, their mother is screaming from her porch, "they just got hit across the face and there is no law against hitting your children in the face." What steps can I take to save my children from this savage situation?