That "crack" you see in the Xray in the back of a child's heel is normal; it is a growth plate.  It is normal to see these "cracks" in the various bones of the foot, and they remain open until the later teenage years.  But if your child is complaining of heel pain, it is important to have him or her examined by one of our doctors.  It could be a condition called Sever's disease (calcaneal apophysitis) which is really no disease at all and very common