I think you'll find that the kids understand what's going on. They are also likely torn because they want a relationship with both parents. What happens between the parents, is the parents business as long as they get what they feel they deserve - parents. For now. At a later date, with more life behind them, they'll be able to better voice what they saw, how they felt, and what it means to them. I wouldn't want to be the mom at that point, but good things can still come of it. It's never too late for that.
I know my son realizes. I know my ex tried to "replace" me with OM. I know the kids feel lied to and betrayed by their mother. I know they also saw my examples and they know I love them and never left them. That's what I was able to do for them, even though at times I really felt like taking a ride on the crazy train.
My son, talking to his uncle a while back who asked about his mom and OM, said that "the guy was ok, but he wasn't my dad."
They know. They also know it's not right, which may play into the concern. But they'll find a way through that with your help.
Enjoy the cabin, Wet. Sounds like fun.
AJ
"Our lives begin to end the day we become silent about things that matter" MLK Put the glass down... "Yesterday I was clever so I wanted to change the world Today I am wise, so I am changing myself."