This is a perennial question. It depends on so many things. There are a couple of verities, however.
1. Once acknowledged, it can't be reversed. "Mom and Dad have special friends...with benefits." Nor can mom and dad do anything other than ask the child to be discrete. Being "outed" is a possibility. So the first question is "how mature and adult is the child?" In other words, can they keep secrets and do they understand discretion?
2. The real question is why you would want your child to know your sexual behaviors. Most kids think "Ewwww" when it comes to their parents having sex. We know we thought that of our parents. Yet our conception was surely not immaculate. There is a tension between being private and being truthful.
What seems to matter most is how sexual matters are generally discussed in the home. Most homes, not only is sex private, but discussions of sex are closeted. Surely in such situations, swinging is just as closeted and needs to stay that way.
What about the alternative? What about the home where discussions about sex were -- and are -- no different than discussions about politics or hobbies or the movies? The picture changes a little bit. Our adult daughter knows that we have special friends, knows the attend adult parties, and know that we have been on a lifestyle cruise and are looking forward to the next. Her view is that we are unique among parents --referenceing her friends--in that not only are we still married to each other, we are still madly in love and show it in so many ways.
The question arises--she has asked--are "you two" (meaning us) so happy and secure because of your special friends? Or are you able to have those special friends because you are so happy and secure? We are pretty sure it's the latter. And while she has no ambitions at present to be in the LS--her current boyfriend is quite enough for her--she's paying attention to the fact that we _are_ content and comfortable with our sexuality.