I know my son knew his mother had a "boyfriend", although I'm sure he doesn't know how many. While a teenager, he started to act up one day, so I had to explain to him that our "friend" was "our friend". That everything was okay, and that he didn't need to worry about anything. He was okay with that. After he grew up, he wanted a DNA test. He made it sound like it was an exciting ancestry thing. I did it, and he is my son.
My youngest is the one that would not be mine, but she's never done the test. I think she probably knows her mother was fucking around too with the same guy my son was concerned about, but again, not how many guys, although she has met the guy that is her father, she just doesn't know he is. He was my wife's first boyfriend after we were married.
The funny thing is, the whole family knows my son's wife cuckolded him. And there is some doubt in my mind that his youngest is really his, and to add to that, the child doesn't look like him.
I've never talked to my son's wife about it, but I have let her know that she is still part of the family, and still loved.