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Enable dynamic requests and one-click responses (e-mail/notification/SMS) for additional screen time
Microsoft Family Safety in Windows 8+ does this best. When time is up for a child on a particular device, you should offer the option to request more from the device they're being blocked on, which then notifies a parent and enables a one-touch response (via SMS, e-mail, or (preferably) Android/iOS mobile app notification), including the ability of the parent to grant a variable amount of additional time (whether time allotment per day or extension of allowed hours). It's a huge pain to temporarily adjust the allowed hours (and then remember to change them back) under time supervision for one-time exceptions to house rules. Android has allowed simple interfaces within the notification tray for a while now that you can use to augment push notifications - it'd be perfect for this!