Using this website or the app on your phone, you define a policy for your child. This may include when they are allowed on devices, how much time they can spend per day, how much more time they could earn by doing useful things, and what apps are homework versus playing games. Any devices with ScreenLimit installed will follow the policy when the children use them, giving them notifications as their time runs out.
The ScreenLimit app is a single install on each device whether it's for parents or children. You can select whether the device is for a child or a parent in the Devices section later.