Samsara, this world we live in, is thus. It is a world filled with great beauty and sometime great ugliness, human beings who live in peace and those who kill. Sometimes innocent children die. Today is a day for tears.
The true evil doer is greed, anger and ignorance, mental illness. That is the disease, that is the real culprit.
Yet, the Buddha also Teaches of a View of Views, free of birth and death, beyond violence, with no separate killer or killed, no gain or loss in Wholeness ... and so no hearts which can be broken thus. We must keep the View that these little children have returned to This which is never left. May their parents and loved ones also find Peace.
Even though there is This beyond violence and death, nothing to break or repair ... hand in hand, we must work hard to fix this world and make it better for the future, ending the violence, poverty, sickness.
All at once. This is our Practice.
I would suggest that the best place to make a contribution is to a charity dedicated to ending gun violence or violence in general. Here are some suggestions ...
Brady Center To Prevent Gun Violence
Futures Without Violence