Luis, you are missing the point with this. I have never discussed intimate details of what goes on with my spouse with my children. My spouse has never discussed it. We have no desire to discuss it with them, and neither would they have any interest in hearing it.
Yet our children KNOW we are monogamous, loyal, faithful and committed to each other. They are very secure in that, and never worry during a heated discussion, that one of us will leave.
Children are extremely observant, and often hear and see things that adults did not mean them to see or hear. They are adept at gauging nuances of behavior, even if they do not fully understand it. It is amazing, how many children realize that a parent is cheating before the other spouse even realizes it.
I think it would be an interesting survey to poll swinging spouses, and ask how many intended to keep the knowledge from their children....and how many were successful at it.
But the van Dams are "bad" for doing the exact same thing you did?
Just because you do not approve of their alleged lifestyle, the rules are different for them?
One last thing, no one is "cheating" in an open marriage.