The children will have experienced your abuse as a trauma and it sounds quite prolonged, he also abused them which is traumatic in and of itself. They’ve also had a family breakdown which again is traumatic for kids even if him leaving makes them physically safer.
I wonder if the anger and violence from them is flight/fight response, which is a trauma response and out with their conscious control. The episode with your son certainly sounds like that. They won’t realise they’re responding from a place of trauma, because their home life is normal to them.
I’d look at what’s happening before things kick off, are there things that might remind them of the violence in the home (even just a change in atmosphere). Your son is probably better behaved when your ex is around because he knows your ex will be violent to him if he isn’t, you may find him decompressing once your ex leaves because he’s been keeping it together - kids mask a lot when they don’t feel safe.
In terms of intervention/support does their school have a nurture base, inclusion in nurture groups is a low level intervention which they might accept and which might create enough of a sense of safety for them to think about counselling. In terms of parenting, I’d be keeping things as calm and routine as you possibly can which will help their stress levels. They won’t have much capacity for stress without tipping over into fight or flight so everything you can do to reduce stress around the home is a good thing. Can you look at contact arrangements so your ex sees them away from home, and home becomes a fully safe environment for them. Also have a look at Therapuetic Parenting - it’s a way of parenting designed for kids who have been adopted which focuses on strengthening relationships and reducing shame rather than behaviour management. It’s highly effective with kids who have experienced trauma. The temptation is to crack down, set hard boundaries etc but your kids need to rebuild their relationship and develop trust in you again.
There’s no point trying to address behaviours while your kids are dysregulated - they literally won’t be able to process what you’re saying to them. Wait til things are calm and then talk to them from a place of care and concern for them - eg I was worried when you did X, I could see how angry you were, and it hurt when you did Y but mostly I’m thinking you’re struggling to cope with big feelings” or however you would speak to them.
You might find they respond to sensory type activities, eg movement (jumping on trampoline, running, throwing things), having a bath, things that are tactile, fidget toys which engage their mind and body.
It’s very hard because our instinct in danger is to remove ourselves, while your children need to to draw closer so you’re going against your own instincts in caring for them. Have you sought therapy for your own trauma? I’d start there and would continue it as you try to care for your children, so you have space to process the impact on you.
Try to avoid taking a purely behavioural approach, you can’t address their behaviour without considering the impact of trauma on you all.