I think, and I realise this is very unpopular, but I think that it comes down to choice.
A person who stays in an abusive relationship has made that choice. - now, please understand what choice means. unless you are tied up and locked up, you have made a choice to stay. the reason you have made that choice is likely to be fear, terror, feeling like you don't, in fact, have a choice. but you have decided to go back home after going to the supermarket, to go back home after work, to go back home after the school run... Now I know that the reason people do that is that they are generally so ground down and afraid that they just are not thinking clearly. I know that. They have a reason for their choice that seems clear to them and seems to be the best and least dangerous one.
but. a child in that situation has no choice. none at all. nowhere to go. no refuge to flee to alone. no way to get themselves out of that situation, no matter how hard the choice to do so would be for them. if they had it.
They are trapped in that situation because they have no way of leaving without a parent.
So if the parent keeps them in that situation because of their own fears and feelings and worries and belief that they have no other choice, then there does come a point when someone outside the situation may have to step in.
If, however, the abuser has been removed from the family, and comes back to cause harm, then there is no way that the children should be removed or it should even be threatened. That's stupid. That's like blaming the parent if a burglar got into the house and taking the kids away because someone broke in. Stupid. The police should just prosecute like they would anyone getting into a house where they had no business being.