We used to be very furtive about our poly relationship, and hid it from our parents. If someone was coming over, we'd "parent-proof", hiding anything that might incriminate us, like the latest issue of "Loving More" magazine.
One day, after an outing, my Dad dropped us off at our house, and came inside for a minute to use the bathroom. When he came out, he was holding one of my wife's lesbian magazines in his hand, and said, "Uh... I was wondering..."
Oh well! The cat was out of the bag (or the lesbian was out of the closet!), so we told him. His first response was to be completely baffled. Nothing negative, just confusion. After that, we went ahead and told my Mom. She was a former hippie anyway, and was very supportive. We should have told her from the start. She encouraged us not to get married, but just to live together. We didn't listen to her, and got married anyway. Sometimes when we're dating people, we look back and think she was probably right!
It was very liberating to be "out" with my parents.
Poly seems to run in the family. My sister is in a long-term FMF marriage, and my wife's 1st husband is also in a long-term FMF marriage, with kids from both women. My sister and I were raised in a very healthy sex-positive environment. My Mom just had live-in boyfriends until she died - such a hippie!
The kids don't exactly know the whole story, but then most of them are still quite young. When we've had partners who lived with us, the kids didn't question any of it - it was just the family. We homeschool, so they know that in many cultures people have more than one spouse. We have many gay/lesbian friends, so the kids know that a man can have a male spouse, and a woman can have a female spouse. We also have a gay couple who are "uncles" to the kids. At least 3 of our kids have shown signs of being straight and at least 2 seem to be mono (one is married), but we are still supportive of their lifestyle choices.