Given the wide variety of quality around vendor provided controls I wonder if a better route (sorry) is the router. Figure out what the MAC address is of the device and see if there are ways to basically shut off internet for that device after X minutes consumed. Maybe you'd have to write some code and run that devices DNS through a Raspberry PI or something, I don't know if routers have parental/time based controls like this. This doesn't stop them from playing games w/o internet requirements though so obviously some physical monitoring is needed.
A more manual way would be to figure out at what % battery left is appropriate limit of their screen time and then charge the device just to that level (like 8% is 30mins or something). Probably not great for the maintenance of the device but solves the problem.
Another option is to write an app of good quality that does exactly what you want, makes it easy on the parent to manage, maybe tracks usage over time, etc.
A more manual way would be to figure out at what % battery left is appropriate limit of their screen time and then charge the device just to that level (like 8% is 30mins or something). Probably not great for the maintenance of the device but solves the problem.
Another option is to write an app of good quality that does exactly what you want, makes it easy on the parent to manage, maybe tracks usage over time, etc.