I know this is an old post but I wanted to provide an update to this.  As of today, it appears that disabling the supervised restriction "Allow modifying Find My Friends settings" no longer greys out the Location Services switch for devices running on 13.1.2.   iOS devices on 12.3.1, or older, will continue to have the Location Services switch greyed out.  Hopefully another work-around can be found in the near future!   ... View more