What are the causes of children's cough?

Patient's question:

Patient Age: Four and a half years old
Detailed Condition and Purpose of Consultation: Generally in good health, but experiences a bout of coughing after waking up in the morning. The sputum is yellow with a greenish tint and very thick. Inquiry about the cause.
Duration and Onset of the Current Illness: Lasting for about a month, with no significant improvement.
Current General Condition: Coughing occurs every morning, rarely during the day. Morning cough is accompanied by dry retching.
Previous Diagnoses, Treatments, and Outcomes: Taking expectorant and cough-relieving medication helps when symptoms are severe, but they always recur.
Auxiliary Examinations:
Other: Yes

Doctor's answer:

Hello, if the sputum has a foul odor, it should be considered a lung abscess. It is recommended to go to the surgical department for a detailed examination.