Soreness in the rearfoot of children isn't common, but when it does occur, the most common cause is a condition called Severs disease. It's not a real “disease”, but it's the label which has unfortunately stuck. It is properly known as calcaneal apophysitis. It is a issue in the growing area at the rear of the heel bone. Because it is a problem, of the growing bone, the problem is self-limiting and will no longer be a concern when the growth of that bone has finished. It is more common around the ages of 10-12 years.
The classic sign of Severs disease is discomfort on exercise and soreness on compressing the sides of the back part of the heel bone. In the beginning the pain is relatively minor and doesn't affect activity very much, however later it will become more painful and affects exercise participation and may even lead to limping. The exact cause of it is not clear, but it is clearly an overuse type issue since it is more common in kids who play more sport and more frequent in those who have got a higher BMI. Kids with tight calf muscles may also be at a increased possibility for the chances of this problem.
Generally, the management of Severs disease is load management. The child is urged to remain active, but simply decrease exercise levels to a level that can be tolerated and not too uncomfortable. A soft heel pad in the shoe might be useful to cushion it. Ice soon after sport can also be helpful to help the pain. If the calves are tight, then a stretches needs to be started. At times foot orthotics may help if the arch of the foot is lower. On rare occasions a splint may be used, and all sport ceased until it heals. By the mid-teens the growth area that this occurs at merges with the rest of the heel bone, and this ceases to be an issue at those ages.