This shooting is unprecedented in the US as far as the number of children killed, and will mark a point in history. Considering I don't even watch the news or go to any news web sites, if I hadn't heard about it on HN, I probably wouldn't have even known about it to be honest.
For a shooting, yes. But the 1995 Oklahoma bombing included 19 young children among the 168 killed, including all 17 children in a daycare in the building.
This level of death is, sadly, not unprecedented. And I guarantee you that in 20 years it will be even less remembered by the general public than Timothy McVeigh is.