You are probably thankful for your job, and have a interest in your parents business succeeding.
The people your parents hire don't share that opinion.
I honestly believe that employee attitude reflects management leadership. You need a carrot and a stick approach, whether it is a restaurant or a factory floor.
It needs to be a pleasant work environment for those that work, and a unpleasant work environment for those that don't. Overtime, and easy work assignments are reserved for those that work hard. Reduced hours, shitty shifts, or termination are available for those that don't.
Most people aren't going to be self motivated for a job that doesn't accompany a very comfortable life style.