Lollipop I haven't posted on your thread before but I just want to say, you can still protect them even when they have to see him.
I know that sounds insane, and you have been absolutely screwed over by so many professionals, but there are experts in this area who can advise you, who have worked with women and families suffering from domestic abuse.
Staying with him would mean your children never having a safe space. The longer you stay away, the more they will realise that with you it is always safe. They will have a period of time to recover between visits. They will have somewhere it is always safe, and they are loved in a way that is good and pure. It is not enough to counterbalance the risks they face with him, it is not a permanent solution, but it is something that can be survivable, for you and for them, with the right supports for you, until things can get sorted out.
If he gets visitation, and the abuse escalates, there are steps you can follow to reduce the impact those visits have on your children. You are not the first woman who has been failed by the system, and has had to use different strategies to protect her children while waiting for the courts to do what they should. There are professionals who specialise in supporting mothers to work with their children who have been affected by DV, and in some cases still are being affected, and they can help you make this the least worst it can be, which will definitely be better for them long term than you staying.
If he physically harms them in his care, you will be there to document immediately and make sure it never happens again. I know that is an unthinkable thing to have to deal with, but it is true. If they are emotionally abused, you will be a safe space for them to repair and recover, and there are people who can help you minimise the impact it has on them. I know theraplay is something that has been used successfully with dv.
There isn't a good answer to any of this. He is an evil manipulative and clever man. But in the long run, you as there mother will have more influence on them than him at his worst, and you can help them recover from what has happened. I can't imagine the situation you are in with such a young family, and such constant fear and aggravation. I would echo others about contacting Womens Aid, but I would strongly recommend you start looking at the supports that can help your children, as that will hopefully make you realise that you have more control than you think right now- not as much as you should, not enough to fully ensure they are safe, but possibly enough to make them safer than seems possible right now. As others have said, you owed nobody anything except yourself and your children. I wish you such good luck and all the strength you need x