I wish there was an easy answer. You are right about one thing, the kids are going to do what they do, no matter what is in any agreement. That can be good and bad... just try to remember that they are going too do what they think is in their best interest, in the moment! And that might not always match up with what you would prefer. Just so you know!
OK, so here's the thing. I can understand not wanting to shock them, but I don't know that you can tell them ahead of time. If you tell them ahead of time, you are setting up an "us" vs. him thing, since they won't be able to tell him. Ideally, you would tell H, and then the two of you would tell the kids together, but I don't have a lot of confidence that that would go really smoothly, either.
I'm most worried about your D. What's her relationship with the two of you like?
