I carried my .357 while fishing in bear country this summer, but I doubt anyone would shoot at a couger while it’s in the midst of an attack on a 5 yr. old.
That’s a very tough call to make without being there and knowing whether there was a safe shot possible.
If nothing else, a shot into the ground to scare it off. And if you’re close enough to hit it, you’re pretty much at point-blank.