The latest Apple operating system IOS 12 has a free feature where parents can control the apps a child uses and restrict the amount of time on the device.
The latest Apple operating system - IOS 12 - has a new Screen Time feature where parents can control the amount of time the child uses the device, and also certain apps during a day. It is very easy to use once the device has been updated.
While it is possible to set limits without discussion with the child - it is a good opportunity to have a discussion with your child about what they think is appropriate usage and also share your preferences for the amount of time they spend outside, talking with friends face to face, playing with lego, playing sport, reading paper books etc.
It also allows a parent to see what apps the child is using during the day and control these limits from your own IOS device if you have family sharing turned on. To learn how to do this watch this video or do a search on Youtube.