See doctor. It is rare for children to be experiencing chest pain of a cardiac cause. Most often when they complain of chest pain it is due to chest muscles or even an infection like pneumonia or a condition such as asthma. However any complaints of chest pain should be evaluated by a pediatrician. If there is pain or feelings of being light headed during exercise please see your doctor right away.
Many causes. Most children have "heart" pain from something other than his/her heart. Bones can hurt, there can be reflux, or even breathing problems. But complaints of heart pain need to be taken seriously as some very rare, but often deadly, heart conditions can cause pain in a child. Please discuss your specific case with a physician to determine the best course of action.