Although i can't quote from it, I remember that Nomo's talk with his kids was really well done, so that should be a great resource for you.
When I had to deal with this 5+ years ago, younger S was 6, older 12. One of ways we couched it with S6 was to talk about it in terms of a time out for H and me (like what sometimes would happen with Ss), not able to get along and trying to figure out a way to make things work (which at that point I thought really was the case; although for young children it may not really matter what that truth is...) And at this point, they will not understand or care about any notion of who wants what...they will only want to be reassured that you will be there for them...
We took turns in our home, so this did not involve Ss moving from place to place...but I don't think that makes your initial message any different.
And there's no sugarcoating it Puddle (which i'm sure you already realize)... the way I felt about having to have this convo made any R convo with H seem like a day in the park...
