Two words...
Parental Controls.
Built right into every iOS device. Work pretty good too...
This is NOT a technology issue. This is an issue about children being curious and what we, as parents, do about it. When I grew up, it wasn't phones but nudie magazines. I'm sure before my generation it was something else. The curiosity will ALWAYS be there. So will the temptation. This isn't about technology.
BTW, this isn't directed at you but those who see technology as the issue.