Seriously, it's pretty far-fetched to think some child willingly just picked up some gun to shoot herself just for the fun of it, even if she thought it was a fake gun.
Not when you consider the huge difference in weight there would be between the real gun, and the wiimote accessory. All of that unexpected weight, especially considering the child would have had to drag the gun off of a table, could easily have caused her to say, lose her grip with her index finger still on the trigger. The gun spins around the loop, the child loses her balance and begins to fall, the trigger gets pulled by the weight of the gun pressing against her finger...
All in the blink of an eye.
I fully agree that the parents were completely irresponsible here, merely by PURCHASING either the gun, or the wiimote accessory considering their strong resemblance to each other. Nevermind that the gun was placed in reach of the child while loaded, and that the child was being allowed to play lightgun games at all when at an age where such confusion could occur. That even begs the question of whether or not the child had ever been instructed to make sure what she was reaching for was a toy!
There were so many ways that this could have been prevented, the majority of them the responsibility of the parents. But it is also important to note that at least one failure in this catastrophic mess was the responsibility of the manufacturer of the wiimote shell, which as mentioned, was jet black with realistic markings.