I told my daughters (then 18 and 20) the full truth. One had already suspected, and the other (D18) already KNEW, having seen her mother with the OM and it had been eating her up inside, trying to protect me from it.
My son (then 14), I said something like "Do you know why me and mommy have been arguing?"
"Kinda," he said.
"Do you want to ask me anything?" I asked him.
"Not really."
"Well, I want you to know the truth, and I also want you to know that I would NEVER lie to you, nor have I ever, okay?"
"OK."
"Your mother has a boyfriend. Married people shouldn't have boyfriends or girlfriends while they are still married -- it's wrong. I want her to stop it, and she refuses to, and that's why we're having problems."
"Oh," he said. "I think I know who it is." -- and then he proceeded to say "It's that guy from the gym, isn't it -- that young guy."
Kids aren't stupid. They hear things, and we do them harm, usually, if we don't have them hear it from US, first, in a loving and age-appropriate manner.
