One more thing. This:
The poor kids have been subjected to too much and the last 6 months have been peaceful (relatively) and I hate that I made them have to witness this...
No.
Your ex made them have to witness that.
You didn't.
And the silver lining to what happened is that while it must have been incredibly traumatic for them, they now have an opportunity to make up their own minds about their father. I know you've been very good at not badmouthing him to them, at letting them build their own relationship with him independent of what he's done to you.
He has shown his true colors and he has shown his true ugly colors to
them. It is tragic, it's horrid, it's ugly. But now they
know. And as painful as it is, they have the truth and they will get help in therapy to know how to handle that truth.
And none of it is your fault.
None of it.