It is not fun to go through this with your kids. I am sorry you are all three struggling.
For myself, I found that being as honest as I can has been the best thing I can do for my kids. I decided on that course of action after talking to a lot of adult friends whose parents had divorced when they were kids. My kids don't yet know about his dating (the boys are too young; and I just can't inflict that kind of hurt on my daughter), but they know I am hurt and angry. They know he isn't spending the kind of time with them that other kids' dads do, even the ones who are split up.
I haven't told them what he's been up to, but I have promised them to be honest if they ask me questions. I do not feel like I would be doing them a service to keep their story a secret from them. So far they haven't asked.
This is not to tell you what to do. I sympathize with you, and as hard as it is, I'm almost glad that all this is happening when the boys, at least, are too young to understand. I'm only telling you who I've decided to be, as a parent. I decided when I was a teenager that life would be easier if I were the same on the outside as I am on the inside, and to the best of my ability I try to make myself the best person I can figure out to be. That is my goal as a parent, too. To balance compassion and honesty. They are both critically important -- for me, it is the only way I can teach my kids to be people who perpetrate the kind of hurt their dad has dealt out to his family. If my children grow into adults who honor their commitments and deal compassionately with the world, then I will have succeeded as their mother.
Me42, H40 D12, S8, S7 A revealed: 7/13 Sep 4/14; Agreed to D 1/15