I don't know much about WAY after DDay. But, we told our now 17yr old son about 3 weeks post DDay. It didn't really happen the way I wanted, given so close to DDay. I'm not proud of some of the things son saw early on. I totally lost control and trashed all the wedding pics, any reminder of OW, and ran over an extremely expensive tri-bike with FWH's truck....in front of H. Son saw the house and the bike. It was a double betrayal and our son knew the other family involved, well. He knew we were going through something rough, prior to disclosing, and had his suspicions. FWH didn't want to tell him anything. Our marriage counselor suggested telling son, just because things were so hard at home. Again, I'm really not proud of that.
Ideally, I had wanted to sit him down with FWH and myself - the three of us together. As it happened.....FWH told him before I arrived home. So, I found a son VERY angry at his father when I arrived. FWH was all...."it is going to be fine" at that point. I wasn't so sure. I think we had had maybe one MC session by then. Things were still VERY emotionally charged. I let son know that I wasn't sure that everything would be fine, but that no matter what happened to us and the marriage.....nothing would change our feelings for him. At that point, post DDay, we knew we could come together there. We all eventually learn our parents aren't God, but that day son lost that for us both. FWH was our son's hero. That was sad to watch. We did offer counseling for our son, and he had a few sessions.
There was some acting out early on....but our home life was very much in turmoil then. We didn't fight in front of him, but there were many a difficult discussion behind closed doors. There were also a few times when I was absent, our son felt responsible for monitoring FWH's actions. For example, there was a time when FWH was on the phone with a male friend of our marriage (someone who had been through an affair also) behind a closed door, and son thought FWH might be on the phone with OW and was extremely upset. This doesn't happen now, but was a thing for awhile.
Son has seen OW once since DDay in our small community, and verbally lashed out at his father over it. It seemed like he was triggered by it. There were some moments where son was super disrespectful to his father. FWH handled it. We all talked about it. Our son isn't the kind of kid that would typically do anything like that with us. He's an only child and we talk through most everything.
Nearly 3 years post DDay and son has had a front row seat to reconciliation and recovery.....and all of its highs and lows. He HAS seen us do a lot of hard work. It has affected how son treats dating relationships. He has a greater maturity and integrity in these relationships than I've seen from his peers. I hear about his friends dating multiple girls at once. Son does not. Cheating isn't an option to him.....or at least that what he shares with me.
Our parents don't know. All are frail and elderly. And, we're not close. But, everyone else knows. All of the people we're closest with know. Their support has been immeasurable. Having all these folks around us, has helped. As time goes on, it seems that no one is untouched by some form of infidelity whether in their relationships or their parents. Not what anyone wants to acknowledge, but that has been my experience with our group of friends. Son has confided in a few friends about it. For me, there was some shame attached to people knowing I was the BS. Not so much anymore.
If I had it to do over again, I would have tried much harder to keep my emotions in check early on. It sounds like you've been successful there. I would have also been in the room to explain with FWH. I think, if we could have managed that together at the time, it would have helped son to KNOW that with regard to son, we'd always be committed to working together.