We didn't have a talk, more an argument at 10pm where ex gave me an ultimatum and I took the "Get your ass out of this house, I'll talk to a divorce attorney in the morning!" option. The boys were 5, 9 and 10 at the time. My eldest wanted to be on his own, and the other two and I went to bed and cried our selves to sleep and then the next morning we all started to look to the future with optimism. My children have amazed me in how well they have dealt with a lot of crap (And there has been a lot!). I don't know if they have done so well because I have not let this get to me or because we have such good communication between the kids and I. They only see ex eow and hardly talk to him in between visits so I am the one who influences them the most. Up until the last 4 months I would always say I love you and Daddy loves you, and really reinforce that, but I have stopped after doing it for 2 1/2 years.
As to when I talked to them about what divorce would mean, I could only say positive things about it, I saw no drawbacks and so my children didn't either. I was really cheerful (surprising as the night before was tears) and felt like a weight had lifted from my shoulders.
I imagine as you have been living under so much stress you will all feel so much better when you make that physical move, or your ex does. I would emphasize that positive, and stress how dad will not be far away, and now both parents will be less stressed, more relaxed and happier, and still want to spend time with the kids. If you can do this without bitterness and bad behavior on both sides then that really is what you should aim for. I do really wish ex didn't resent me or blame me for the hole he is in, but he does, and he has stooped really low to satisfy his vindictiveness. I would do anything to stop that.