Guns are little more than a tool, completely inanimate on it's own. Physically, it requires an operator to use it, and subsequently, to kill. In these cases, the child was the operator. Under this reasoning, I can 100% declare that guns don't kill children, it's the one handling the gun that does the killing (child or adult, it's irrelevant).
Looking at places like Chicago, the run-down areas of LA, and my old neighborhood near Sacramento, I wouldn't be surprised if a majority of those deaths involved gang activity. Though I shouldn't underestimate the woeful lack of education in the topic of guns, I wonder how many kids in middle school or even high school know to check if a gun is loaded, or god forbid, to know not to play with one.