TUNDE
It's not unusual to have this type of complaint with sporting activities. A visit to your Physical therapist will be very helpful in analyzing the body movements and ways of unloading the stress from mucle groups involved after their sporting activities. The Physical Therapist can also help in terms of instructions on self remedy such as cold or hot packs based on assessment of the situation. Although you did not specify how long they've been involved in athletics, the situation might get better as they get used to the activities provided there is no underlying problem.