is about what happened with my parents. They accused Mono, my bf of abusing my child. It might be a good read. It turned out okay in the end and has made our family stronger than ever. I decided early on to let them see our boy but if they said anything to him about Mono then they would not see him. I got him looked at and there was nothing of concern from our family doctor so it was left unfounded.
I didn't talk to them, and they didn't talk to me for over a year. Eventually they started talking. I think they realized that they could not prove anything and while they didn't understand they could accept that and work on being close to us again. They have never been as close. Something died during that event. I don't tell them as much any more and we are all more private. They don't need to know what we do anyway.
I really admired Mono during this time because he didn't change his persona. He was the same as he always is. He was the same to them too. He said that there was no reason to be different because he had done nothing and he could understand that they would be concerned. Poly is weird to most people.
I think if I were you I would just laugh it off and not take it seriously. Pass it off as their ignorance and loving concern for you. You know this won't happen so why sweat it? i can assure you that poly kids have got it made! Provided that their parents NRE doesn't create neglect that is and provided that all partners consider the kids with every relationship move they make. Kids thrive in poly! That has been my experience.
Have a look at some of the threads tagged "kids" "children" "family" and see if something helpful comes up.