My husband travels for business as well.... the kids were used to him being gone, but not on the weekends, LOL!! MLCers always seem to want to "tell the kids", especially if you are not solidly in their corner.... sometimes they use it against you, if you try and set a boundary of "maybe I won't want to be with you, you lying, cheating, basta**!" and sometimes, I think they just use it for dramatic purposes and as a bellweather to see where you are at as far as jettisoning them forever, hahha!
I told the kids, in a very lengthy and serious conversation, that Dad was "working through some things" and needed some time apart to "think about stuff" and my 10 year old daughter piped up and said "Oh, is he depressed?"
so that is how I explain it, especially since it is true. I explain DEPRESSION as debilitating, and irrational.... since it really is.
I did use the term midlife crisis with my teenage son, but with my daughter, it is depression. Her therapist also explained to her that DEPRESSED PEOPLE cannot care properly for others..... they are very selfish in their behaviors. This seems to help, even though it still makes her sad. At least they understand, to a degree, that Dad is not a bad person, and that he can't help it. They also understand that we have to live our lives, and we're sad he's not in it much, but it's because he's depressed.
I do not say "Dad isn't here because he chose to spend Christmas with a F'ing ow and her daughter cuz he's a worthless piece of sh**" even though I have moments where that is the overwhelming feeling... it's not true. I simply say "Dad should be here. I can't believe he's not here. It's not right that he's not here..... but he is depressed, and depressed people do terrible things and don't understand the consequences... I'm sorry."
I also tell my kids that if Dad ever puts them in ANY situation (meeting OW on the sly) that makes them uncomfortable, that they can slip away and call me and I will come get them. They need to know that they can go with Dad, but if it's dicey, they don't have to stay. Good luck.