Final answer:
The nurse should first confirm the child's identity and allergies before administering the medication.
Step-by-step explanation:
The action the nurse should take first when assisting with the administration of the child's initial dose of parenteral opioids is to confirm the child's identity and allergies. This is important to ensure that the medication is administered to the correct patient and to prevent any potential allergic reactions. While assessing the child's pain intensity using a pain scale and checking the medication expiration date are also important steps, confirming the child's identity and allergies takes priority to ensure safe and accurate administration of the medication.