I can't speak for iPhones, but Android has a file system and the ability to create directories. I think it would be weird if Apple products didn't have that as well, even if it's a bit hidden. I don't have any form of evidence to back this up, but I think that this has more to do with apps, not the device that kids use. Having everything be an app makes it seem like you have multiple independent things on your device, and the device itself is nothing more than a way to connect to those apps.