read terms and conditions of policy. you may have cover for casual or occasional employees.
However, there is a difference between an employee and an independent contractor. Do they men get paid by the hour, is PRSI paid by your parents, who controls what they do?
Could they be contractors? If that is the case, then your parents liability towards them is not as high as towards an employee.
The important thing to remeber, is that just because a person is injured, it does not automatically follot that they are entitled to compensation. Sometimes people can the the authors of their own misfortune.