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As for the little girl, this is something that will traumatize her for the rest of her life, and most definitely affect her mental well being.
I think the social reaction often makes these sorts of things worse, like a kind of reverse ostracism.
Similar things can be seen with rape cases, where some women suffer mental anguish for years despite getting lots of care, while others shrug it off and are able to move on a lot better. Ironically, the victims seem to suffer
more if they live in an advanced Western environment where rape is culturally less acceptable, as opposed to the victims in areas where women have less/no rights. Putting it very crudely and insensitively to deliver the message: most of the suffering seems to be as result of social conditioning/brainwashing to maintain cultural expectations, and that conditioning is strongly reinforced after the event. Of course I don't disagree with maintaining good standards, but we should also be aware of side-effects.
As Lisa Simpson would say: "I don't want your pity!"
However, the Americans will probably serendipitously shrug off yet another gun accident, and the girl will make new friends with several of her classmates, who have also shot various family members and pets.