You are in the situation we were in for six months, though my husband did mention "should I need a separation" numerous times. During this, we didn't tell our children and we just plowed on.
When my husband said he had to have a separation, I made him tell the kids. I told him that we were parents together, 100% behind our kids, but I didn't want them to think I was at all OK with splitting up the family. Telling the kids was difficult, but we've done a really good job so far at putting the kids in our best interest. In this situation (sharing the house), I'm the one getting the shaft but my kids' comfort is bigger than my own right now.
He just told them that he was really unhappy, that adult relationships are complicated and that he had to get a little space. That he loves them more than anything, but needed to do this.
As for GAL, dry_heat, that's something that I always thought was a major positive of our relationship. Turns out that we were TOO independent of each other. Our mutual friends kind of dropped away after we moved two years ago. We used to get a weekly babysitter, but couldn't afford it once we got this house, so we tended to go out without each other more often than together. I stay at home with the kids and for my personal definition I'm active in city politics. City meetings happen in the evening. My school volunteering and my non-profit work happens while the kids are at school and are so separate from my husband. Our problem wasn't that we didn't define ourselves but rather that we didn't define us as a couple. We were more parents and individuals.
Though my husband is young for a MLC, I think he's got a bit of that going on too. He's quite successful, has a nice family, enough "stuff" without being really material and achieved most of the things he wanted. Then I think he looked around and said to himself, "This is it? This is what I'm going to deal with until retirement/kids are old?" Most of his old hobbies, his passion for volunteering and politics just vanished. A lot of his friends moved back to the Midwest and he didn't pursue friendships, having just a few friends from work. (And our dental hygienist, his EA.)
He took up marathon running and has run three in recent years. This would get him out of the house and away from me for hours and hours. I don't run. The only other thing he'd do was go to the gym. I'm more of an outdoors person than a gym-rat and the hiking, camping and the like that we used to do just stopped. Honestly, I stopped suggesting we go kayaking, hiking, etc. because he'd never want to do it. The only things he wanted to do was yard work (landscaping, major stuff) or go to the gym. He lost his mojo. Now I understand. He disagrees with the MLC stuff, saying he's too young, but he's an "old" 33. Has been his whole life.
Me: 42/H: 37 T: 10 years/M: 8 D9, S8 Bomb: 7.23.07 Separated: 1.20.08 D Final 3.19.09 Affair started in '05, found out parts in 11/07. They married 11.26.09