Think of yourself as a chaperone, a driver who has their students' best interests in mind. You could someday save a child's life by protecting them from predators! The interviewer wants to see how you would react in this type of situation, because it's possible to occur. You can respond by saying, "I would first talk with the person to verify if they in fact were the child's guardian. If they cannot prove so, I would contact the police and do my best to keep the child and others on the bus safe."