If the parent of an existing employee ever called me concerning his or her son or daughter's job performance, I'd give them a warning but if it ever happened again, I'd fire them on the spot. This isn't elementary school. We have a job to do and businesses to run.
Whether or not any parents ever contacted your company about something it’s quite likely you’d not hire someone in the near future ~ at least not if you were the average company STILL LAYING PEOPLE OFF!