Cough is a helpful phenomenon - it is the body's natural reflex to irritation in the throat. There are two kinds of cough - one which is wet and associated with mucus (productive cough); and the other which is dry and there is no mucus (unproductive cough). It is important to differentiate between the two, because the treatments are entirely different.
In the case of productive cough, coughing helps in removing excess mucus or phlegm which has come up in the back of the throat from the lungs or the nose and sinuses. A common cause of coughing in the night is mucus dripping from the nose into the back of the throat. Expectorant syrups can be given to the child to help get rid of the phlegm. Note that productive coughs should never be suppressed since they serve a very useful purpose. Night-time irritation can be eased by turning the child on his side or his front. A doctor should be consulted in case the cough lasts for more than 48 hours.
Unproductive coughs on the other hand serve no useful purpose, and can be very irritating to a small child, sometimes to the extent of preventing sleep. In this case, cough suppressants can help ease the discomfort.