In Philip Greenspun’s Weblog, he notes he just ran into a family of circus performers where the children were far more well-behaved and polite than others he has met. Since the children were “home-schooled,” Greenspun hypothesizes that spending time with a high percentage of adults might be good for manners.
Well, aside from the obvious “It depends on the adults,” I think it would be far more likely in this case that two other factors come into play: social space and professionalism.
Social space: Living your life in an RV with your family would likely encourage some sort of accommodation, and civility is an accommodation conducive to living with others.
Professionalism: I don’t know if the kids perform in the act, but their parents likely do. That’s a pretty early introduction to the working world, dealing with strangers, and taking your actions seriously.
So, what’s going on with these children? I’d say their polite manner is helped along by their self-discipline, which is a core requirement for civility.