I dont really see how parents sex lives could benefit or harm children if it was kept private. It isnt really any of their business. Chances are they dont want to know anything either. I didnt when I was a kid.
Kids are pretty observant and get more so as every day goes by. According to the article, the author (man) indicated that his wife had been on dates 4 times so far during that month. Later, it was also stated that he took his turns as well.
You're right in that it's not the kid's business, but it won't be long, if not already noticed by the kid(s), that one of their parents are gone at least twice a week... Probably only a function of time before one gets up in the middle of the night to see one of the parents roll home looking like one does after knocking boots.
Kid won't really get it (unless they are let in on the deal), but eventually they will.... That is, if the marriage remains healthy and survives.
That said, what these (or any) consenting adults do in terms of their sex lives is completely their business, but I do believe that it is foolish to pretend that their kids won't notice something as time moves on.