A dubious finding? Sure. But least PolitiFact California's fact check did not try use the rationale that might have made all victims of gun violence "children." But the PolitiFact video used to help publicize the fact check (narrated by PolitiFact California's Chris Nichols) went there:
|How many teenagers in the background photo are 18 or over, we wonder?|
Any parent will tell you that any child of theirs is a child, regardless of age. But that definition makes the modifier "children" useless in a claim about the effect on children from gun violence. "Children" under that broad definition includes all human beings with parents. That counts most, if not all, human beings as children.
Nichols' argument does not belong in a fact check. It belongs in a political ad designed around the appeal to emotion.
The only sensible operative definition of "children" here is humans not yet of age (18 years, in the United States). All persons under 18 are "children" by this definition. But not all teenagers are "children" by this definition.
To repeat the gist of the earlier assessment, the claim was misleading but PolitiFact covered for it with an equivocation fallacy. The equivocation fallacy from the video, featuring an even more outrageous equivocation fallacy, just makes PolitiFact marginally more farcical.
Edit: Added link to CPRC in first graph-Jeff 0735PST 1/12/2017