How to prevent the phone from connecting internet?

I have a BQ Aquaris X2 phone, already unlocked bootloader and rooted. I gave it to my parents but they keeps watching inapprociate videos and news. So I need to prevent the phone from connecting internet , even after factory reset (they can play games, watch videos offline, no need calling or messaging). I have thought some solutions:
- Find out which system application that helps the phone communicating with cellular network and wifi, then uninstall it: I've searched but can't find any...

Read more