As i said last night, I speak from experience, my dad used to hit me and my brother to discpline us, then he moved onto our mother when she tried to stick up for us, then he just used to hit any of us whenever the mood took him. She walked out on him eventually, after several failed attempts, leaving us behind as she had nowhere to go to start with. After a while she got herself sorted and we lived with her again, it took a lot of courage for her to leave such a violent, manipulative man. He was very good a playing dad and being the perfect husband, when he wanted to, but behind closed doors things got worse and worse.
When my mum left we were very angry at her for splitting up the family, but in time we came to realise that it was for the best, (and I honestly mean that) we could all say, do, think and wear whatever we wanted without fear of it being 'wrong'. We also got to know our mum properly, which has been fantastic, it's been nearly a decade since all this happened and some of what happened will stay with me forever, like the guilt of not being able to protect my mum and brother and some of the horrible things I saw. But on the plus side, I have got to know my father again, he is re-married and much happier and calmer now. So just because your kids love their father, doesn't mean it's right to put up with all of this, they will be upset, of course, but sometimes people are happier and have better relationships because of a bit of distance!
Things will be very hard, but they will get better, (my mum promised me that when she left and although I couldn't see it at the time she was right) but act now before some real damage is done.