There are numerous causes of heel pain in adults, but in kids the most common by far is a condition called Severs disease. Since this is an injury to the growing plate at the back of the heel bone, it would not happen in adults. The most typical age of onset is around the early teenage years or a bit before. When we are born the heel bone expands from two regions, one being the major area of the heel bone and another being the growth zone at the back of the heel bone. These two pieces of bone are divided by a zone of cartilage. Severs Disease occurs when there is too much force on that area of cartilage.
The leading causes are simply too much activity done to rising amounts so that the bone doesn't get a chance to adapt to the loads which are placed on the bone. Most typically the child is participating in a lot of sports activity, typically on hard surfaces. Limited calf muscles can also be commonly present. The primary symptom is pain around the edges of the heel bone at the back of the heel and soreness on activity. Increasing the amount of sporting activity also can make it worse.