You can limit an app from the Insight, Activity and Rules pages, and for newly installed apps, even from the notification!
The general rule to limit an app is to find an app in one of these pages and tap on the 'Rule' button
To set up limit for an app,
Tap on the child and go to the 'Rules' page
Tap on the app that you want to set limit for.
Tap on 'Set Limit' and choose the time limit for weekdays and weekends.
Don't forget to tap 'Save' before you leave!
Alternatively, you can setup limit for any app by going to the Limited apps page,
Tap on the child and go to 'Rules' page.
Then go to the 'Monitoring & Apps' tab that is on the top right.
Under the 'App Rules' section, tap on 'Limited apps'
Tap on the 'Add an app' button and select the app you want to limit.
On the bottom sheet, choose the time limit for weekdays and weekends. Additionally, you can also modify the routines during which you want the app to be usable.
Don't forget to tap 'Save' before you leave.