I have small kids, niece, nephew, parents, in-laws... they all use iphone + other apple products. I am glad that I do not have to explain what not to do to any of them!
Why the h*** would you authorize your small kids to download porn from their iPhones? I'm not a parent, but if I got my kid an iPhone, I'd never lie about their age so that they could access adult content through the App store. You might be unaware of this, but there's porn available all over the Internet through browsers, so you better have that talk you thought you could avoid.
As to your parents and in-laws, they understand the whole concept of sex. They would know what they were about to access based on the warnings about explicit adult content.