You need to scan the legs from the top down to the feet so make sure you’ve exposed the legs appropriately.
Check for muscle bulk and wasting, leg alignment problems such as knock-knee or bow-leg deformity. Ask yourself if the legs look the same length. Does there appear to be a flexion contracture at the knee? Look for swelling of the knees. Check the feet, including the soles of the feet, for local causes of a limp such as a verruca or a foreign body. And always scan the nails and skin for features of psoriasis.
Check the knees for warmth, comparing one side with the other. When assessing for an effusion, ask about discomfort and scan for facial expression. Gently press down over the distal thigh to move fluid into the joint from the suprapatellar pouch. Compare each side. A patellar tap suggests an effusion, but for a small effusion a patellar tap may be absent. It may be helpful to milk fluid from one side to the other. Again, compare one side with the other.
Normally a child of school age is able to reach their heel to their buttock. Compare both sides and check that the knees fully extend. It is very helpful to compare each side for symmetry as normal ranges of movement vary with the child’s age. Firstly check full flexion, then flex the hip to 90o and check for external, and then internal rotations. Check also for full extension at the knee.