Dr. Greene's Answer
Coughing at night can come from several sources, including post-nasal drip (either from allergies or an infection–cold or sinus infection). The mucus drains down the back of the throat while the child is lying down, causing the nighttime cough.
The nighttime cough can also come from allergies–usually dust mites in the pillow or mattress. Or, nighttime cough can be the only symptom of asthma.
When the cough has lasted for less than 10-14 days, it is usually a response to an infection, but even so, it could be reactive airway disease or asthma. If so, an entirely different type of medicine is used to treat the cough than if it were caused by a cold. (Colds, however, are the most common trigger at that age.)
When kids are coughing enough to need two pillows at night, it is wise for them to be checked, even if they are fine during the day. Even if nothing turns up, it is the wisest choice.