Have you heard this from your child? I have and my spring schedule is filled with children with the same problem. Many times kids who play cleated sports such as soccer, lacrosse and baseball get an inflammation of the back of their heel bone. This is known as Calcaneal Apophysitis or Severs Disease.
It is a constant source of frustration for parents and children alike. We wonder 'Are they really hurt?' or think 'How can it be so painful? I never saw them injure it!". It is typically not brought about by a 'injury' or 'sprain', but rather is an overuse injury. It is caused by the repetitive pounding to the open growth plate at the back of the heel. It is often made worse by a tight heel cord, cleats, hard or uneven ground, and drastic changes in exercise regimen.
So how do we treat it? Typically we do not need to take them out of activity, but by working with a consistent icing/stretching regimen, the correct inserts or cushions for your child and sometimes bracing the pain can resolve quickly. In particularly stubborn cases they may require immobilization in a walking boot or cast.
The earlier you seek treatment, the faster it will get better. NO, it won't go away on its own! May people will say "Oh, it's just growing pains' but I do not believe any such diagnosis exists. If your child is complaining of pain, then they need to be evaluated by a foot specialist as soon as possible to avoid more damage and inflammation to the area.