Fainting is extremely common in childhood. In a small number of children it is a sign of heart disease, but that is really very rare. In most it occurs when they are ill, under physical or psychological stress, when they are hungry.
An attack may be caused by extreme pain, fear, or stress resulting from an overstimulation of the vagus nerve, which helps control breathing and circulation. A child could faint from prolonged coughing or by straining to defecate or to blow a wind instrument. Another cause could be remaining in a stuffy environment without enough oxygen. In addition, standing still or standing erect for long periods of time can cause fainting due to blood that pools in the leg veins, cutting down on the blood available for the heart to pump to the brain.