I don't know how much of a Star Wars fan you are, but there is a line from the movies that has always stuck with me:
"Many of the truths we cling to depend greatly on our own point of view."
I think when you tell your children their birth stories, what matters most is telling them the story as you experienced it at the time.
You can even be very specific: "When I was headed to the hospital, I remember thinking..." or "When they put you in my arms, I felt..."
Those things are still true. The knowledge you have now changes the context around those memories, but it doesn't erase what you felt when your son was born.
I don't think there is any reason to disabuse them, or yourself, of the magic you genuinely felt in those moments. The birth of your child was still a magical event for you, even if you later learned there was betrayal occurring alongside it that you didn't know about.
His affair changed what you know about that period of your life. It doesn't get to steal your most precious memories of your son.