Yep . . . just as I thought. Your source uses ages 0-19 as "children and adolescents." Guess what happens when you lump in those extra two years of 18-year-olds and 19-year-olds, who are legal adults?

You also lump in a disproportionate amount of street gang-related shootings, which have different causes and different solutions, and make it look like suburban teens are uniquely at risk from firearms . . . when they're not. The vast majority of firearm homicides in this country are suicides, and the next-most involve street gangs and the drug trade, and involve young men with criminal records killing other young men with criminal records using illegally-obtained pistols.

That doesn't mean the death rate of children due to firearms is high, it could just mean there isn't really anything else that kills children i.e. they mostly have parents looking out for them, don't have to work dangerous jobs, and haven't lived long enough for lifestyle factors to take effect yet.