But he won't because neither of us can face telling the kids and he refuses to tell them anything other than that we BOTH want this.
I don't think it has to be either or with regards to talking with the kids (and it took me along time to get here). What about something like "Sometimes in a marriage b/c of frustration and unhappiness a break is needed. This way we can decide how to proceed and make the best decisions we can for this family." Now, I know you don't think a D is best and that's not what you're putting out there for the kids. I also know you want him to take "ownership" of this mess b/c he's the one that leaving. Now answer me this, how does telling your kids that help them? This is hard and it's a personal decision we each have to come to and live with. What's right for me may not be for you. I'm just throwing in my 2 cents.