i have not read all the thread.
My DP and i had an affair, but ended it and a few yrs later ended our marriages separately and then re-connected so its not exactly the same.
However my eldest dsd knew about the affair, she was party to much of the fallout at home when his now ex wife found out, and was used very much as a pawn by her mother. ( her description not mine)
My eldest dsd never truely believes that we ended the affair, and was slightly tricky about us being together however she slowly has come round, and now we have a good relationship.
my dsd2 has always been ok with dad having first a GF and now a partner.
BUT OP....we took it soooo slowly, we split from our spouses, we had our own homes, we dated discretely for over a yr before we told children we had even 'met' someone, and we then did some very careful introductions.
tthe biggest moment was when we took all four girls on holiday ..that was the first 'blended family ' moment, and we only did a week and then we both had a week on our own with our daughters so they had space and time to discuss/talk process how they felt about us each now having a 'significant other'.
it was 3 years before we moved in together, and by then my dd1 and dsd1 were at uni, and dd2 and dsd well into gcses...so older and perhaps cus they had their own lives to a degree less bothered by us.
In fact dsd2 once said she was so glad i was in her dad's life as it stopped him focusing too much on what she was getting up to !
it took time, patience, and both of us never badmouthed our previous spouses, we tried to be honest but we also did not really want to discuss our previous affair with them. My dsd1 did say she now realises that the original affair was not what caused her parents to splitt and that they are both so much happier apart, but she also recognises now that has taken her over 5 yrs to understand.