Googles Family Link app is a service that lets you monitor your childs or teens location and smartphone usage remotely from your own handset. The app was originally designed to monitor children ages 13 and younger but a recent update now includes teens older than 13.
Today, the Family Link app has a parent and child/teen version of the app. Monitoring activity, setting limits, and locking a device works pretty much the same with children. For teens, the parent and teen accounts must both agree to monitor the teen. The Family Link app can be disabled from the teens end but the parent will receive a notification when that happens.
Google Family Link can also push teacher-recommended apps directly to the childs handset and Google is going to expand these kinds of features like screen time, usage limits, and remote app-installs to Chromebooks in the future.
Family Link: Parent iOS | Android
Family Link: Child/Teen Android