IOS (iPhone)
Screen Time. You can enable Screen Time on your kid's phone and password-protect the settings so they can't change them. Or, you can also manage the settings remotely by setting up Family Sharing. Work on using screens intentionally and mindfully, and help your kid learn to regulate their own use when you're not around to do it for them.
Key features:
Usage report. A daily and weekly readout of the time you're spending on your device. You can see exactly how much attention you and your kids pay to app categories such as social networking versus, say, reading and reference.
Downtime. Turns the phone off during a specific period of time -- for example, 9 p.m. to 8 a.m.
App Limits. Sets daily time limits for app categories such as games and entertainment as well as for individual apps.
Always Allowed. Lets you choose which apps (for example, music apps) that never turn off - even during downtime.
Content & Privacy Restrictions. Controls what your kids can see (such as mature content) and do (such as download and delete apps). Also puts limits on how much information third parties can access about your kid.
Android
Digital Wellbeing. Unlike the Screen Time features in iOS, you can't enable Digital Wellbeing settings on your kid's phone and password-protect them. Instead, Digital Wellbeing is designed for individual users to customize their devices to their own needs. If you're an Android family, you can discuss and try various features to make the phone work for you -- instead of the other way around. (If you want to have more control over your kid's Android phone, check out Google's Family Link parental-control app, which allows remote monitoring.)
Key features:
Dashboard. Graphs the time you've spent in individual apps and lets you set daily time limits for apps that keep you hooked longer than you'd like (for example, 15 minutes tops on Snapchat).
Do Not Disturb. Silences your device entirely or allows you to specify which alerts you want to see (or not).
Notifications. Personalizes your alerts, so you can snooze them and schedule them at a convenient time.
Wind Down. Automatically turns your phone grayscale and enables Do Not Disturb at a time you specify.