Their happiness seems to defy all odds and is nauseating to boot!
Things didn't work out so peachy-keen for my ex. He had an extramarital affair with a woman who was also married with kids. The two of them got married as soon as their divorces were final. It was about as messy as things can get, given that two families were destroyed in the process. Though my ex has since tried hard to get my kids to know and like his new wife, they have so far (to my knowledge) refused to have anything to do with her. I don't think he even mentions her name in their presence. And I'm told that his wife's son is none too happy with his new stepdad.
Mind you, these kids knew about the adultery and saw how it tore up their families and hurt the other parent. I wonder if the kids of the couple you described were aware of their affair at the time that it was happening. I think children prefer to look the other way if possible.