As Amy says, your mom is perfectly capable of seeing you alone without her husband. Your children should absolutely not be forced to see him if they don't want to, even if there IS an adult in the room - and as you've said, he's had outbursts even when you were there to protect them, just not of this magnitude. Your children need to feel safe and know that you will support them.

Regarding your mother: with his rage problems, it is possible that he behaves the same way toward her, and that his behavior is escalating, so you should still try to maintain a relationship with her for the same reasons. On the same token, she has also chosen to stay with a man who behaves violently towards her grandchildren and has assaulted and abused at least one. I know people remain married for many reasons; however, she has to make her own decisions, and accept the consequences of them - which for now include that she may see her children and grandchildren less due to her husband's actions.