Many times, this is an inflammation of the growth plate in the heel, or calcaneal apaphysitis. This can be treated many times with ice, rest, Children's Motrin and heel cushions. There are variations of these treatments, but usually, the child outgrows this when the growth plate closes.
Check out the different heel cushions on these various web sites. I like the silicone shock absorbing kind.
Vivian Abrams, DPM