People abuse because they were abused, and that's the only way they can regulate their emotions. Not that it means the guy is not at fault, but just to give insight into why this happens (I feel as though people like him deserve sympathy until they begin perpetuating their abuse on others, at that point, sympathy disappears and the situation must be dealt with).
Those kids are going to need years of therapy, or it's depressingly likely that the boy is gonna grow up just like daddy and beat his wife, and the girl is gonna grow up just like mommy and get knocked-up by the same kinda guy as daddy. It will perpetuate itself endlessly, as it always has with the kid's dad, and his father before him, and so on.
It sucks. It sucks a lot, but the mental health situation in this country is so much worse than the physical health situation. The stigma around getting help for your various psychological ailments is utterly deplorable, and it keeps people from getting the help they need, and it either exacerbates the problem, or it perpetuates it on future generations. It's like having a broken arm and being too ashamed to get it set by a doctor; you're only making it worse.
My brother is so ashamed that people will find out about his OCD that he is relapsing and as a result, everyone knows about it. If there wasn't so much stigma, maybe he could be more open about it (even to his shrink) and maybe he could improve his situation; it's not like anyone we know would judge him about it anyway. It's just sad that we shame people with legitimate, in some cases, life-threatening illnesses to the point where they want to pretend they don't exist, which only makes it worse.
I'm sorry for these kids. I dunno what you can do about it; perhaps suggest some counselling for them and maybe the mother. I would call the father a write-off at this point.