Final answer:
The unsafe task for the volunteer is transporting a school-age client who is in traction, as it requires specific medical knowledge and handling skills.
Step-by-step explanation:
The question is asking which task assigned to a volunteer in the pediatric unit is unsafe. Volunteers typically do not have the training necessary to handle complex medical equipment or situations that require a high level of care or monitoring. The unsafe task for a volunteer in this scenario is transporting a school-age client who is in traction to another department. Transferring a patient in traction involves understanding how to handle the equipment and maintain the integrity of the traction, which is beyond the scope of a volunteer's responsibilities. The other tasks like playing with an adolescent who has sickle cell disease, reading to a child with AIDS, or rocking an infant with croup do not involve medical procedures and therefore are generally considered safe for a volunteer with appropriate guidance and precautions.