Is foot, ankle, or knee pain normal in kids?

No, never.  A child should be comfortable throughout their growing years.  Often times pain in the feet, ankles, knees, and back are an indication of faulty mechanics and the subsequent mal alignment of the bones of their feet and ankles.  In our opinion, growing pains are not something you grow out of as they continue into adult hood in the form of other problems such as arthritis for example.  But fortunately "growing pains" or any other structural or functional problem your child is having is rather easily handled in most every case with a good pair of total contact orthotics.  Your doctor can tell you very easily if your child will benefit from a pair of custom orthotics.