First let me state my assumption. A working child is one who is over 18 (the age of majority) and employed on a full time basis.
They are legally adults, they are employed, they now need to be contributing.
I can't see why one would question whether they should pay rent.
A parent's role is to guide their children to develop into adults who can take care of themselves and contribute to society.
If the adult child is now earning a full time income then the parent has fulfilled part of their role but until that child is out the home and supporting themselves then they are still developing them.
If they live at home without contributing to rent/bond, w&l, food, laundry, cleaning etc then they are taking a major stop on the path to becoming independant. A stop which they may find very difficult to restart from.