What are you getting out of this relationship now, consider that question carefully. What needs of yours are being met here still?.
Your children have never had two parents who can actually work together in a marriage. You always co-operate; he never has done. He blames you and continues to blame you for all his inherent ills so why are all your hopes here seemingly pinned on therapy for him?. You really think therapy will work here?.
You managed to leave him once and you can do so again particularly once there is a plan of leaving safely in place. Your safety here is paramount. Do not keep on teaching your children that he is more important than you or they because that is what they are seeing now. Fear of both him and fear generally amongst many other factors keeps people within abusive relationships and it is not easy to leave. It often takes several attempts. But its bloody hard to stay and he could well put you or even one of his kids in hospital; he is not averse to knocking your eldest one about and that will likely continue as well. One day too, your children will grow up and leave home, what then for you?.
Do you really also think that he should actually spend any time with either child in the long run given that he has been violent towards your eldest as well. All this man cares about is him, certainly not you and certainly not your children either. I doubt very much that he will actually see either child in the long run let alone want to pay for them via child maintenance.
Therapy ultimately will not work because this is deeply ingrained within his own psyche. Look at his parents OP: what lessons did they teach him about relationships?. Chances are that one of them was abusive towards the other at home and that is what your now husband learnt from them. He grew up seeing all that. I am making an assumption here I grant you but I bet your childhood was no picnic either so is one factor in making you far more vulnerable to the approaches and attention from this abusive man.
He certainly should not be with you now but I think its a given he will refuse to leave. You will need to therefore employ legal means to get him out via an occupation order and legal advice with a view to divorcing him. You will also have to start properly opening up to people in the outside world because abuse like this can and does thrive on secrecy.
Your children are not the arbiters of your relationship, you are. Children like seeing both their parents together along with stability but with you two being together they are getting none of that. They have seen and heard far too much already. They are getting a warped sense here of what relationships are all about and it would not surprise me if either one or both of them ended up in abusive relationships. This is no legacy to leave them.
If you've never had a night away from your children as well and have not worked, you sound isolated emotionally as well as physically. When do you ever get a break?. I take it as a given there is no respite care and you cannot rely on your so called husband to be of any use re them either. It would not surprise me if your isolation was deliberate on his part as well; its all been engineered by him to keep you further trapped and controlled.