What you would need to do to make it safe for a 9 year old to shoot an Uzi

You would have to set up some kind of permanent frame that would prevent the kicks from getting loose from the child. So, if you wanted the child to experience shooting the weapon there would have to be a limit to how far the muzzle could move in the kicks from the bullets leaving the muzzle.

Most children don't have the focus to use their strength in the right way and might be scared by the loud noise from multiple bursts of fire. They just aren't prepared for the actual experience of deafening sound combined with kicks like someone is literally hitting their hands and wrists with a 2 by 4 during the kicks of the gun firing. So, to protect the instructor and the child you have to have it within a frame of some kind so it can't kill anyone and stays in the general vicinity of the target.