Yes, “the leading cause of death among children is a firearm” is an extremely upsetting sentence—and also, a sadly accurate one (given that you define children as “humans between the ages of 1 and 19”; infants in their first year die from lots of stuff that doesn’t kill you after your first year; if you include them in this number, then it skews toward premature birth, birth defects, and SIDS).
But, the truly upsetting part is buried in this chart (shown below with a big dumb pink circle to emphasis the “Mechanisms” section), which was an addendum to the original source Schumer’s staff cited.
Population wide, gun deaths are usually ~66% suicide and ~33% homicide. Among children, that’s now basically flipped.
In other words, in America today most gun deaths are suicide, and most adults will die of something else (probably disease). But for kids in America, the leading cause of death is guns, and most of those gun deaths are murders.