While I believe they would all benefit from some therapy, at this point, I don't think they would listen to too much of anything I might mention.
They probably wouldn't want the long sad details. They would want
validation of their feelings, in time they may accept your guidance to the past reality. The value of good counselors is that (1) they've had training, (2) they can share perspectives and teach skills as emotionally neutral experts.
Remember the old story that you can lead a horse to water but you can't make it drink? That may describe your spouse, but probably not the kids, or not all of them.
I have been told to give them space to process.
Space, perhaps, to an extent. But too long or too much and they may lose connection with you. And you mentioned he had told them his biased claims. You don't want their current feelings influenced by your ex to continue for too long that you're part of the problem. (Maybe you were when you were an appeaser, but you're done with that. You'll have to prove over time you're changing for the better just as you want for them.) In other words, be there but not pushing an agenda, get a feel for where they are and do what you can at that level. For all we know they may never want to hear all the details, at least not anytime soon. In time they may see you're not hiding his poor behaviors or 'protecting' him from consequences any more. In time they may listen to your account or proofs of the history. Your calm, stable and loving example
now may prove more than words ever could.
Do you have a counselor? An experienced counselor could assist you in techniques and skills to regain or improve your relationship with your children.