My kids were 10 and 12 when 9/11 happened. You just explain that there are bad people in the world and bad things happen. But that Mom and Dad's job, and their teachers, and all the other adults in their lives are to make sure bad things don't happen to them. And then you hope they don't. I don't know how I'd have explained something like Sandy Hook. I have memories of wars and assassinations and even a vague memory if the Cuban Missile Crisis and I recall being aware but not terribly worried as a kid. Kids are pretty self-focused which helps make them resilient - if it didn't happen to them or someone they know I'm not sure they really feel the impact. To a certain extent you have to count on that.
-Ray