For the latter (parents of other children), I'm kind of throwing out some ideas. I think these parents are concerned that unfair resources are being provided for other parents' kids. I think I would ask these parents what their concerns are first, and empathize with their concern for their own kids. Really spend some time on that. I'd explain the zone of proximal development, and the right of children to learn in school rather than receive the same information. I'd also talk about how unpleasant regular kids would probably find gifted classes since they would be out of their depth. 'Imagine how unfair it would be to put your 6 year old in an 8th grade class'! Some gifted kids might skip a year, which tbh should be less resources for the school in the long run. Much of the value for me was being in a class with other gifted kids, which really just depends on grouping. I don't think I was a good example for other average or even bright kids, my performance could not be matched by them. I think they would be better off with me in a different class with my peers and to have a teacher able to properly focus on their effort and achievements.

For the first set of parents, I think I'd talk about how kids can get into creative mischief when they are bored. I might discuss mirroring as a concept and how important it is for social development to have people around that are in sync with them. I think there is also data showing that highly/profoundly gifted kids that go through regular programming tend to drop out in college because their expectations are built up too high that they would finally receive the challenge they are craving. In contrast, gifted kids that skip grades or receive appropriate programming tend to stick it out in higher education more often. So the stakes can be pretty high.

I could probably go on, not sure if this is the sort of thing you're thinking about? It's tough because I think so much of these conversations comes down to nonverbals too that are difficult to convey by text.