I am sorry lovely I don't know. I would have hoped that the legal system would protect them but as you are seeing it doesn't.
I am pretty certain that you are doing all you can to protect them. The only 2 reasons I know that prevent a court from awarding contact are risk of serious harm to the dcs and that the children seeing the father causes so much stress to the mother that it affects her ability to care for them.
The problem with the second option is that if you seriously lose the plot he will claim that you are not fit enough to care for them.
The first option has to be proved and I assume social services/cafcas have to have real concerns and there bar for good parenting is pretty low.
I realised long ago when going through all this that tho ex was a complete cunt, a shit father, didn't take good care of dd and was emotionally abusive that the law says that a relationship with someone like this was more beneficial to dd than not having a relationship with her father. It's fucking shit. It's heartbreaking and you feel fucking useless. It broke my heart every fortnight dropping her off. Then I worried for 48 hours until she was back with me.
It does get easier though. She came back dirty, hungry with matted filthy hair and a bag full of sweets. She ate mcdonalds and crap from Friday afternoon to sunday teatime. He didn't clean her teeth or bath her. Her clothes would be filthy. And they either watched tv all weekend or went round to his mates house.
I worried constantly what she was being exposed to. She always smelt of cigarette smoke as did her clothes. I bought her a coat purely to have at her dads as I couldn't send her to school smelling like an ashtray.
It got better as she got older. And she stopped seeing him earlier this year. Her choice. She was 12.
I think you have to accept he will get contact. Whether it is supervised contact initially or not is irrelevant because they will move towards unsupervised contact. I think you will drive yourself insane trying to stop it. Unless the police press charges on him assaulting your dcs court won't be particularly interested in what he has done to you as it is the dcs they are deciding on.
I am really sorry I can't be more positive for you. But the reality is he will get contact moving towards unsupervised contact. The only thing you can do is fight for as little as possible and as supervised for as long as possible. I really am sorry 