Last summer I explained, best as I could, to my 12 yo son that the front brake was the stronger one and the one that should be used most of the time. Next time he was out riding his bike (older Kona with hydraulic discs) he decided to test the new knowledge I had imparted and grabbed a handful of front brake to see how fast he could stop, and LO! Over the bars he went!
New knowledge imparted to me: It takes about 4 weeks for a 12yo's broken arm to heal.
My mistake was not emphasizing that one must practice using the front brake to be able to do it safely. We were riding the other day and I noticed he still uses his front brake most of the time, but he doesn't go over the handlebars. In fact, I bet he will never again go over the handlebars again from using too much front brake. I would bet it is a mistake that the vast majority of people will only make once.