I know my ex was having an affair with current wife / long term partner when child was just turned 1.
But - we were not right together. I wasn't behaving well in the relationship due to my own needs ( no cheating or affairs). They are better suited together as a couple. I am not woman scorned.
Ex would never admit it, they already have lied about how the pair met to our teenager.
But it doesn't matter really. The point is one relationship ended because it was not working. That's the true answer in most these cases.
I would only say something - in this case, not my own - like ' it was not what I personally wanted but sometimes people fall out of love.' That is all the details needed imo ( over age 10).
By adulthood then I'd be ok being totally open and giving the full story - only if asked or if for some reason the grown child was throwing alot of resentment towards me over it.
You can tell the truth without lying yet without giving every detail.
I don't believe it's totally in the best interest of child.
When my teenager repeated the story told him about how his dad and stepmum met ( a made up version told him by his dad), I just listened and didn't correct him. What would be the value?