http://www.cdc.gov/injury/wisqars/pdf/10LCID_All_Deaths_By_Age_Group_2010-a.pdf
Over 40,000 people die in the US every year from motor vehicle accidents and about 5% are children. MVAs are the most common cause of accident related deaths in the US and accidents are the most common cause of death in children over the age of one.
Firearm related deaths are much less prevalent although one is too much. But one is too much for unintentional drowning or any other preventable death esp homicide and negligent homicide.
In undeveloped countries illness esp malaria and AIDS as well as malnourishment and starvation are the most common. The five year mortality rate for Nigeria in 1996 was over 50% with the 1 year survival rate roughly the same.
If you want to make noise about GSW's and deaths then make more about all the other causes esp those involving alcohol related motor vehicle accidents.
You can fire back if you've ever had to walk out and tell a parent their child was dead whatever the cause and esp unintentional GSWs from other children.
And yes those two are negligent in the death of that child and should suffer consequences.