Heel pain is such a common problem in young soccer stars between the ages of 8-14. I rarely attend a soccer game for my son (u10) that I don't see someone limping off the field.
The typical cause is an inflammation of the growth plate at the back of the heel bone; this inflammation is commonly known as Sever's disease. It's made worse by the cleats worn for baseball, soccer and football. Treatment is generally pretty straight forward and involves icing, stretching, sometimes bracing and a special heel cup (NOT a gel heel lift). Also, an X-ray needs to taken to rule out the 'bad stuff' such as fractures, cysts, etc. The important thing is seeking treatment early so your little Messi (or Mia) can get back out there and score some goals!