The vast majority of deaths from firearms are not from mass shootings. This is still true if you look specifically at those under 18 - it's down to about ~2K homicides, most of which are the result of street violence. Mass shootings actually kill less children every year than unlocked guns that they get access to and accidentally shoot themselves or another kid.

Mass shootings aren't even 1% of the gun murders. Furthermore, they are mostly a product of our obsession with them. Mass shootings are for the most part elaborate suicides that include making the person famous. If we didn't make a big issue out of them there would be little incentive to do them.

In absolute terms, is it still a small amount? Doubling doesn't mean much if the number was infinitesimal to begin with.