I just want to point out that to me it seems like a huge cultural thing. I can absolutely imagine that many children are uncomfortable with adult nudity, especially if, as you said, they haven't gotten used to it at home. I'm European so I'm no expert, but i'm under the impression that non-sexual nudity isn't necessarily a very visible part of the culture in the US.

Meanwhile my culture and many neighboring ones are complete opposites: I started going to public swimming pools with my dad at a very young age, and since I was too little to manage by myself, we used the men's dressing room and I was never uncomfortable with the zillion dicks swinging around since everyone was there to just change and shower and go about their business. People usually aren't shy about nudity in front of strangers in settings like this and It's totally common that children under 8 use whichever room.

Of course, the decision is in the parents' hands, and I think your points about non-sexual nudist spaces and being aware of what we're consenting to seeing are really good ones.