Is it possible for you to implement a system which gives control of one phones certain functions to another phone as a dad might want to influence his sons use of phone and purchasing and else...
Eg. Son asks for permission via app on phone before purchase is made. So that if a purchase is attempted a notification shows up on the 'master' phone prompting response and validation before purchase can be made on the 'child' phone.
Also notification control for master user could be important... Such that the notifications dont pop up unless they are allowed to do so.
Notifications could include warnings about cost of something and other info about how much the cost is.
Also app installation restricting might be useful.