Final answer:
The priority nursing diagnosis for a child who received an overdose of morphine is 'Ineffective breathing patterns related to central nervous system depression' due to the immediate life-threatening risk of respiratory depression.
Step-by-step explanation:
The nurse caring for a 9-year-old child with a fractured femur and who received twice the ordered dose of morphine an hour ago should prioritize the nursing diagnosis of Ineffective breathing patterns related to central nervous system depression. Morphine overdose can lead to respiratory depression, which is a potentially life-threatening condition where breathing becomes inadequate. Therefore, ensuring that the child maintains effective breathing patterns is critical at this time. Other potential issues, such as fluid volume deficit, decreased gastrointestinal mobility, and altered nutrition, are also important but secondary to the immediate risk posed by CNS depression leading to respiratory compromise.