OT in relation to filtering, but in response to the last post: the child control app I mentioned in my last post (Kidslox) has recently added a useful feature - that of additional profiles. This enables you to provide your child access to select groups of apps at different times of the day.
It's the only app that I could find that would enable us to give my son access to his iPad during school hours and not have this time count towards his daily limits, but these new additional profiles will enable me to restrict certain apps during school time, or only provide access to say messaging apps outside of a set time (with access to the device fully shut down between 19.30 and 7.30).