Hi Penny,
a few comments:
- The app comes with an optional setting that will synchronize only if you are on Wi-fi
- Depending on your network connection some data will be downloaded or not depending on the size see https://docs.moodle.org/32/en/Creating_Moodle_Mobile_friendly_courses#Media_download_for_offline_usage
- Regarding data usage, you have just to sum everything. So if you have a course with 12 resources, each 1MB, it would be 12MB plus additional KBS for the course information (enrolments, sections names, etc..)
My recommendation will be to download the complete course contents when on-wifi and then they should use only the contents that don't have the cloud or refresh icon (that means that they are already downloaded).
If they see a cloud or refresh icon at the right of a module, it means that it will be downloaded or updated when accessing to it.