iPhone has a relatively new feature that allows distraction controls to be enabled on the phone. With this feature, you can schedule downtime (i.e., choose periods during which the phone cannot be used except for phone calls and approved apps) and set app limits (i.e., set daily time limits for certain apps).

You can find distraction controls by clicking on the Settings icon:

And then click on Screen Time:

You will see options for Downtime and App Limits.

If you are a parent setting controls, definitely choose the “Use Screen Time Passcode” option. This will prevent your children from being able to adjust the settings.