Here are a few articles on children and sport related injuries. All but the last couple on the list are devoted to growth plate injuries. This suggests issues regarding the appropriateness of some sorts of high impact (such as heavy bag and makiwara use as well as some tanren) training for younger karateka. Although I do not teach children, when asked I usually recommend the use of lighter target pads with students younger than about sixteen years old. At the very least instructors should be aware of the possibility of these types of injuries.
Have a look: