I have heard that the medical profesion has said that repeated punching/striking of hard objects, like that which is done in conditioning can cause damage over time. I was once told that the conditioning of the hands was done when fighters toughened their bodies to become weapons. However, today, since people live longer, these types of disabilities surface. I think it is no different for a carpet installer, who gets bad knees from their work or a runner who develops bad knees from all the pounding. As far as breaking goes, I am not sure that if fits in the same boat as the repeated forging against hard surfaces. I would be more worried about the possible trauma from all the kicking, not the bord breaking.
Like Mr. Flynch says, no breaking for kids. No way