The problem is, shooting .22's, 9mm pistols, and AR-15's doesn't prepare someone for a 4.5" barreled stubby 9mm that fires 21 rounds per second. It takes adult grip strength to keep one of those tamed.
Here's some footage of one; fast forward to 0:40 and notice how it pushes the (experienced adult) shooter around between 0:40 and 0:45. Also note how high the center of mass is above the grip axis (large rotational torque, which you can see in a couple places).
http://www.youtube.com/watch?v=jI9cKhApwv8&feature=relatedReplace the adult with a young child who doesn't have the experience to let off the trigger if the gun starts to rotate, and you have a recipe for disaster if the stock slips off the shoulder; if the trigger is held down and the gun is uncontrolled, recoil will spin it 180 in half a second. There are full autos gentle enough for young children to shoot under supervision, but an unsuppressed Micro isn't among them.
BTW, it *is* possible for a relative novice with sufficient upper-body strength to shoot a Micro under close supervision:
http://www.youtube.com/watch?v=1nP9oQJfLfk&NR=1but note that in this case, the shooter was an adult with adult upper-body strength in addition to prior shooting experience, the gun was fitted with a suppressor to reduce recoil and increase inertial resistance to rotation, the instructor has the shooter firmly grasp the suppressor (leverage) to prevent rotation, and the instructor has one hand on her shoulder and the other hand floating ready to grab the gun if it starts to get away from her.
Contrast that with the tragedy in the OP. I don't believe it was suppressed (meaning it had the same stubby configuration as the first video above, thus more recoil and no forward grip), the shooter was a small child, and the instructor was nowhere near the shooter.
BTW, for any non-gunnies reading, that gun is as tightly controlled in the USA as a 105mm howitzer or a 500lb bomb. That's not a civilian "assault weapon", that's a NFA Title 2 restricted military submachinegun.