It’s really hard what it is just based on symptom of fever and cough. A short duration (3-7 days) fever is mainly infection and in this age group mostly viral. Viral fevers typically start like this and lasts for 3-5 days but other possibilities needs to be ruled out. Sometimes viral infections can lead to supra- added bacterial pneumonias. Trust your doctor - get chest X ray done to rule out pneumonia. Also do complete blood count and dengue test if that is prevalent in your localities.