after reading your post about, i didnt quite realise it was getting that bad with the father.
By the sounds of it, i would recommend reconsidering the joint custody arrangement. Perhaps talk to the kids if possible, and do document everything, especially when your children have been upset.
If it is causing that much harm, i would find out your rights, because in the long run, its what is best for the kids is what is important.
No child should be suscepted to abuse, and while i know you want your children to be around their father, what you have to realise is right now he is not being a 'father'. He is being a monster abusing his kids, and just how you would protect your children from a monster, you need to protect them from him.
Not necessarily forever, but just while he is causing them harm.
I would recommend carefully planning your steps if you do choose to do anything, i dont mean to scare you, but im sure you well know divorce can get messy when it involves children. I would say, figure out your rights and if you have a solid case, then talk to him. Explain to him that you do not want to continue to leave the children in his company when he is abusing them. Ask him this…. and really push for an answer. Ask "how would you feel if someone else treated you this way", (or, "how would you feel if someone else treated our children this way", then tell him, they are just kids!
But dont leave them in this situation. It will destory their self esteem and they may end up hating their father, which is not a good thing.