Unfortunately, Apple has not provided any way to force location services to be enabled through MDM; MDM can merely request that the user enable it. The one exception to this is to enable Lost Mode for supervised devices. This will lock down the device totally, and automatically enable location tracking. While you won't be able to use Lost Mode for location monitoring while the user is actively using the device, if there's ever a situation where a user loses a device or has it stolen, Lost Mode is the perfect feature to both track it down as well as make it (temporarily) unusable. Lost Mode can only be disabled through MDM as well.
