Maybe I'm being naive, but my son is 6 and I'm separating from my husband, (or at least trying to) and I don't think at that age they fully understand the whole concept of marriage etc. DH has had his own room since my kids were very little, (they are now 3 and 6) and so they've never really known DH and I to share a bedroom or be very close. He's also only home on weekends and an hour in the morning on weekdays so they don't see him much as it is.
We too are separating because he is just a not very nice person to me or the kids (not physically, but almost as hurtful emotionally, and I'm just plain not happy, we've tried counselling several times). I was going to tell my children than "we" are getting another home, and then we'd all go there and stay over a few times together, then back to our regular home, and then gradually DH would stay at the other home more and more often until it becomes comfortable, and we start calling it "daddy's house" and "mommy's house". I am also hoping DH will still come over every morning so I can leave for work and he can continue to take the kids to school, and that we can continue to take them to their Saturday morning activities together, at least for the first while.
Well, this is my idea, not sure if DH will go along with it but he probably will.
I just thought this would be easier than a big "daddy's moving out" and trying to explain why, more of just easing in to a new routine. They will ask why daddy is staying at the other house, and I'll say that me and daddy argue too much when he stays here (which they know).
So, not sure if it's a good plan or not, but this is the best I've come up with to ease them in to this transition.