You would do well to gain some self-awareness OP.
How on earth do you know she has no idea?
That's nonsense. Unless you are talking to her regularly, you cannot know what she knows.
And the very telling part of your latest post is revealing
As I explained above it wasn't just any fling.
For you, perhaps, but for him - yes it was.
If it wasn't he'd have chosen you.
And you don't want to let him get away with it.
I have just regretted that I have let someone get away with such level of deception.
So you want to go after him now?
Can't you see the logic-gap in your thinking?
Maybe you should have had your wits about you more at the time and seen the signs he was married. In 3 months there would have been clues, but you probably ignored them.
You chose not to tell his wife because she was pregnant.
now 6 years on when she has 2 children, you're thinking of telling her.
You'd be happy to wreck their lives out of some misplaced sense of 'justice'.
(And of course she may already know.)
If you're still giving this man so much headspace, you should consider counselling. This obsessing is unhealthy as is the way you can't take any honest comments, and only want posters to agree with you.