10 year old son had brief sharp pain in chest. afterwards started to have pain in right leg below knee. he says it is better when rests for a period.

Not likely serious: A 10 year old with a brief chest pain and a pain afterward in the right leg below the knee is not likely to have anything serious like a blood clot traveling to the lung unless he has been at bed rest or immobilized for several days or had recent surgery.**But if he's still having leg pain and points to his calf, or if he's significantly more tender in that calf than the other>go to ER to be safe.