Please note: steps one and two of the above video are out of date. The tutorial will be updated shortly.
Open "Apps & Safety" from the home screen, enter your parent password, and tap on "App Limits & Schedules".
Tap "Add a Schedule", give it a name, then set:
The days it applies
The available time window
The daily time limit
The "daily limit" is the total time your child can spend across all apps assigned to that schedule.