I bought them Kindles instead - the parental controls are much better on Kindles than they are on the iPads/Phones.
Since we got them the kindles they are pretty much not allowed on my iPad and I almost never hand them my phone to play with.
Additionally you can put content filtering in place at home to keep them from inappropriate sites via opendns.
https://store.opendns.com/get/home-free
Change your DNS servers on your home network to opendns and setup your content filtering profile to do that.
Also - once you setup the OpenDNS profile for the filtering - setup a DENY filter for some of the common DNS servers out on the Internet (like google's 8.8.8.8 and 8.8.4.4 or Verizons 4.4.x.x) as well as your ISPs default DNS servers. By denying these DNS servers they can't easily redirect their DNS to something other than the OpenDNS servers.
You may need to learn a little bit about technology or find someone you can pay to help you with it if you're not tech savvy.