Pretty straight forward..
You take a wifi enabled exercise bike.
It tracks exercise intensity/ duration
Pedal for 25 minutes at a certain intensity, per 1 day for base computer privileges.
The PC will have a PSU which does not turn on until the Kid has exercised for a minimum of 25 minutes on the Wifi bike.
And then You have a basic vending machine you fill with snacks..
Any time the child wants a snack, the vending machine polls the bike.. If the kid pedals the bike another 10 minutes, he could get a mars bar, or a bag of chips..
This stuff works with lots of lab animals.. don't see why they can't make this straight forward adaptation to small-humans.