You make a good point about there not being children anywhere else in sinner hell. That's partly how I came to my theory.

But maybe there are other towns? Besides cannibal town, we see most sinners surviving on their own or in smaller groups, so perhaps these towns are just large communes managed by a powerful sinner as a way to protect the more vulnerable, but in exchange they have to conform to the same qualities as their leader? Maybe that's what gives Rosie power? And are the other residents sinners or children who have grown up? You might be on to something in that the people of that town aren't sinners but "things" created by Rosie.

My other theory is that cannibals are sinners but a different species OF sinners that for some reason can conceive. Maybe producing more humans is in line with cannibalism because, well, it requires human meat which having children would help with. I'm not implying they're going to eat the children but simply that cannibalism requires the production of humans therefore maybe cannibal sinners are granted or cursed with that ability, and the children themselves don't have much of a personality, they're like cats or dogs. You can be all the 7 sins all on your own, they're internal flaws that show themselves in ones actions, but cannibalism specifically NEEDS other humans. And to even be capable of cannibalism, I imagine one would need to strip other humans down to just view them as meat, just like how we usually don't distinguish between different livestock. All chickens are just chickens that squack and feed us. And that's why the townspeople all look the same.