Apple should really do a better job with this. This is one area they can still make real improvements in as the iOS ecosystem matures.
There should be some kind of 'NSFW' or 'Mature Content' API as part of the OS. Parents could set it via Parental Controls.
- That would allow apps which necessarily show mature content to be part of the AppStore.
- It would also allow apps which can show mature content (browsers, wikipedia apps, reddit apps, etc) to have a sensible, consistent filtering setting. I find the idea of some content being filtered out by default just wrong - the NSFW content is still valid content, and you shouldn't have to dig through settings for every app to discover if there's any content silently missing.
- Since it would be managed by the OS, parents can enforce it via Parental Controls. None of these apps can do anything about the kid just turning the filter off themselves.
- You could even make it more granular - maybe kids could even request exemptions for certain websites or content within apps, in the same way they can ask permission to make purchases (the parents' devices get a push notification, and they can allow or deny it). Maybe the kids are doing a school project and need to access history websites with some graphic images, for instance. The OS remembers the decision, and allows access to the content next time the app asks. The apps would provide a short description of the content, and possibly a web URL of the content so the parent can preview it.
Then we could put this issue to rest, once and for all.