Remove hotspot facility on child phone

Contact your cell provider and have them remove the provisioning on your child's phone for personal hotspot.

The Restrictions option on the iPhone allows you to control a number of items that you would want to prevent a minor from using. However, I don't believe the Personal Hotspot feature is one of them.

You can verify this by tapping on Settings > Screen Time, and checking out the various options that are available to you to set.