Android/iOS: The DinnerTime app is the simplest solution you'll find for parents who want to give their kids time-outs from their phones and tablets. The free app currently works only for Android for kids, but parents can use either Android or iPhone.
Here's how it works: You first install the app on the kid's Android device and choose "kid" when prompted. From the kid's device, you send an SMS to the parent's device, who is asked to link with the child's. Finally, you will have to give the app administrator access on the kid's device. It's all detailed in this instructional video if you have any trouble. There's one catch, though: the parent device needs the ability to send and receive SMS since that's how the gadgets are connected. The kid's device doesn't need phone capabilities.
Once the two devices are linked, the parent gets three options: "Dinner Time", "Bed Time" and "Take a Break". By default, Dinner time is set to one hour and Bed Time is set to 21:30-07:00, but both of these can be changed. Tap either button and the kid's device is locked down for that time. Take a Break lets you tap to lock and tap to unlock -- simple as that.
You can currently link two parents to each kid's device, and two kids to each parent's device -- so essentially a family of four is covered. The DinnerTime Plus app expands this to five kids with an in-app purchase ($1.99), and also enables blocking specific apps -- however, DinnerTime Plus is only available on Android right now for both parents and kids.
DinnerTime (Free) [Google Play Store]
DinnerTime (Free) [iTunes App Store]