You don't want an extension -- those can be easily uninstalled.
The app a number of my friends have on their phones is called Android Parental Control. Unfortunately, Google Play is, well, blocked on the school computers, so I can't research it.
I did find a few results on Google for Parental Control, though:
http://www.androidpit.com/Top-Parent...ps-for-Android
http://getandroidstuff.com/7-best-fr...-control-apps/
http://mamabearapp.com/parental-control-apps-android/
http://www.tomsguide.com/us/pictures...trol-apps.html
You can see if any of these work properly.
If you only have to worry about them using your home WiFi, you might want to set up
OpenDNS which can be installed in your router.
On their website they claim to be used by one in three schools in the U.S. to provide filtering. I can definitely confirm that my school uses it. It's very effective.
