Hi, my 10 year old son has 3 small hard bumps under the skin of his left heel, not on other foot , they are right in the middle of his heel and are quite firm if I press onto them, they are under the skin though, you have to push hard to feel them, they do not look like plantar warts (I have had these myself years ago), just plain, small and hard, what could they be, they are not sore? Thanks Mel