I just had a long reply typed out and lost it.
I was saying that I just went through this with my kids D6 and D3.
We told them together that Daddy was moving out and explained that:
a) we had tried to solve our problems and weren't able to and were making this decision so that there would not be any more fighting
b) that the girls had done nothing to cause the problems and weren't going to be able to fix it-- that it was a grown up problem and that mommy and daddy were learning to make better choices and one of the choices we were making was living apart so that there could be less stress.
c) we explained that sometimes even people who love each other can't fix their problems and explained that the love mommy's and daddy's have for each other isn't the same as our love for the girls and that no matter what our love for them would never ever change (D6 is big on promises and knows that I keep mine so I told her I promised her about this issue and she replied: "like rapunzel keeps her promises" so she gets it)
d) we explained that we will still both see them and explained that i will stay in the house and AH will visit with the girls and call them on the days he won't see them
I didn't get into specifics and neither did AH. The girls will learn in time what AH is like (if he doesn't get help) and they both are already well aware of the issues. We left it vague and presented a united front and told them it was okay to be sad and that we were sad too and that any feelings they had about this were 100% okay.
D6 has talked to me often at night (bedtime) about this and wants me to explain why again and I keep it vague and ask her how she feels. Often she tells me she's angry and I ask about what and we go from there...
So, that's my experience...
I have a link to an article I found on line that helped me plan out this conversation and I also consulted with my T and D6's T about how to tell them and what to say.
Doing it together was really important I think so if you and your H are able to do that that'd be best I think.