That really depends on the reason for the cough. There could a variety of problems and there is no universal answer for it. If the kid is suffering from just an irritation in the throat then you can stop is easily by gargling hot water with salt. If it is due to a cold and the lungs are filled with phlegm then try a cough syrup. Most over the counter cough syrups are useful and they help relieve kids from cough, but remember they are powerful and can cause sleepiness. Vaporub is also another good option and this will relieve some stress in the chest.