Idiots!
Barrel climb is a problem even for an adult.
I've seen a fairly simple, albeit ugly solution: Fasten something to the barrel, chain that something to a supporting framework. There is enough play in the chains that the gun can be aimed normally but not enough play to allow it to point in an unsafe direction.
Let me get this straight
YOU think the problem here is a failure to chain the gun?
Guy should have hung onto the Uzi while she fired it full auto and let her get used to the recoil.
Dumb on his part and the parents' part for wanting their daughter to take a "speed lesson" in shooting guns.
Sad for the little girl who will live the rest of her life reliving the experience of shooting someone in the head at point blank range.
RIP big guy.
If they were going to let someone of that size (I don't really think age is that important here) shoot a machine gun they should have done something about the possibility of the shooter losing control of the gun.
I proposed one solution I have seen done (although it involved an ordinary gun but untrained people), credoconsolans has another that would have also worked.