Final answer:
The nurse should administer additional pain medication to alleviate the child's pain.
Step-by-step explanation:
Based on the assessment using the FLACC scale, the child is sobbing and lying with the legs drawn up, indicating significant pain. The nurse's next action should be to administer additional pain medication to help alleviate the child's pain. Morphine is commonly used for pain management in sickle cell crisis, and the nurse can adjust the dose as needed based on the child's response.