@schmucke, @HeikoGr
I have the same situation and i finally managed to receive the timetable for my daughter by using parents account. She did not register herself for Webuntis so no chance. I made a research on the module and the webuntis API.

By concept the function

getOwnClassTimetableForRange(rangeStart, rangeEnd);

must fail, because the parents account does not have an own timetable. So I used the function

getTimetableForRange(rangeStart, rangeEnd, id, WebUntisElementType.STUDENT);

The trick was to determine which “id” is to use. The id is meant as student’s id, in my case of my daughter. So I had to make a detour via

const studentList = await untis.getStudents();

I made a debug output of studentList to console and searched for the name of my daughter and found the id I needed for the function above. This list is very long because it showed all students of school (more than one thousand).
Afterwards I could use the function and received her timetable successfully.
It only worked by login with username & pass not by qrcode and only by hardcoded id because I did not find a filter method for the studentList, e.g. by name, etc. If available it should be possible not hardcoded but by config - Param.