If they are old enough to be carrying around an iPad then they are old enough to be responsible for the content they access. If they are not old enough for the content, then perhaps they shouldn't be given an iAnything.
This has always been a big concern of mine. I'd like to keep my kids safe, but I also want to keep them intellectually safe, as in I want them to be able to use the tools they need. I took two approaches. First, I vowed to keep ahead of my kids in this area of technical literacy. That was not easy to do and I may have finally lost that now that my oldest is in college.
The other approach I took was to educate my kids and teach them how to use technology responsibly. This is a pretty risky method, because kids don't always use their education the way us parents would like, but so far, I think it has worked.
I suppose I won't really know if it worked until the kids are in their 30's, finally telling stories of the things they got away with.