Many families are providing phones to their children so you can stay in touch and keep up with the times. Typically these are added to a family plan where all devices share the same monthly data. With this great power comes great responsibility, many children (and adults) don’t understand mobile vs wifi data or get addicted to their phones during times they should be sleeping or doing their homework.
There are several solutions available, some you can implement in combination with each other for very effective controls.
iOS and Android
Manage the time your kids spend on their tablets and smartphones.
You can easily set daily time limits, reward additional time with completed tasks/homework, block schedules like sleep time and homework, immediate pause, approve new apps, see which apps are used the most, and access their web history.
More advanced to block Internet access on a per app basis. You can restrict YouTube if not on WiFi or only allow Maps/Hangout access. NetGuard displays all of the apps in alphabetical order and next to each one is a toggle for WiFi and mobile data. Disable access to whichever one you like.
Android, iOS, Desktop
Block apps, filter websites, block and mask out profanity & pornography, monitor their time usage and social media, provide you with alerts and reporting.