The most common cause of pain in the knees in an active 12 yo child is Osgood Sclatters disease. It is an unfortunate name as it is not really a disease but an injury at the insertion of the Quadriceps tendon as it attaches to and pulls/tractions on the bone. At times of growth the attachment site on the lower leg bone is vulnerable.
This is not a ’growing pain’, but rather a condition that is provoked by activity in a growing child ( running, kicking, jumping). The condition runs a limited course and settles once the growth period ceases. No treatment is required but activities may need to be reduced/modified for a short period.
Often a bony bump persists after the acute period because the growth centre of the bone at the stressed tendon attachment has caused new bone to proliferate. These bumps are not a problem at all but are merely cosmetic and rarely seen other than by a trained eye.