Final answer:
The first action the school nurse should take is to assess the most severely injured child and ensure emergency medical assistance is called.
Step-by-step explanation:
Upon arriving at the playground where an emergency has occurred with several children injured, the first action the school nurse should take is to begin assessing the most severely injured child. This initial step is crucial because it allows the nurse to determine the severity of the injuries and to provide immediate care that could be lifesaving. While assessing, the nurse can also instruct bystanders to call for emergency medical assistance to ensure help is on the way. Understanding the cause of the collapse or checking the equipment for safety hazards may be important, but they are not the priority when immediate medical attention is required. The focus should always be on the safety and health of the children involved in the incident.