I'm struggling with this myself. We are telling my son on Friday that my wife is moving out next Monday. I am in the same situation, and will not lie to him and say anything like "Mommy and Daddy..." This is my wife's decision to move out, and she needs to shoulder the responsibility. Not to blame her, but to say that "Mommy has decided that she wants to live someplace else."
There is a good book out there called Kids First. I think there is a website as well. It tells parents what kids want to know, and what they don't want to know from the kids point of view. It was helpful in making my wife realize that what was in his best intrests, isn't exactly fair for either of us on in our best interests. She was able to see my view, and agreed to have our son stay in the home on school nights as a means of giving him some stability.
