** If I change the config.txt file to represent the internal address, our parents cannot access individual events on the calendar because the script looks for the internal address **
Have you tried setting the calendar_url and calendar_admin_url variable in the the config.txt file to a relative address (eg. /cgi-bin/calendarscript/calendar.pl) instead of a fully qualified URL?
If it's the file paths which is causing the problem, you can try uncommenting and setting the $BASE_DIR variable in the calendar.pl and calendar_admin.pl files to the proper file path.
Dan O.
------------------