I doubt anything else will be said about this since no one is pursuing criminal charges. A few things to note though. The big dent in the bullet doesn't appear to be new as it isn't shiny. The boy's thumb and forefinger were injured. The primer strike looks too pristine to be from a pebble or nail.
My thoughts after seeing this:
The boy was holding the previously misfired cartridge between his thumb and forefinger at the base and striking it on the ground. Cartridge went boom. Had he not been holding it at the base, he would have lost his thumb and forefinger instantly.