Final answer:
The nurse advises the mother to bring the child to the emergency department if the child has a persistent barking cough at night.
Step-by-step explanation:
The nurse advises the mother to bring the child to the emergency department if the child has a persistent barking cough at night. This symptom suggests that the child's condition may be worsening and requires immediate medical attention. The persistent barking cough at night is characteristic of croup, a viral respiratory infection that affects the upper airway and causes inflammation and narrowing of the airway.