In addition to shoes that bind the foot and change its shape, we typically put kids in shoes that are too stiff to allow natural flexion. Typical running shoes are produced for a man weighing 150-plus pounds and a woman weighing 130 pounds or more. Many of the same materials are used in the construction of shoes meant for kids who weigh a fraction as much. And the smaller physical size means that all that stitching and those material attachment points net a much stiffer shoe than the adult versions. Kids don't have the physical weight to flex these shoes.