This discussion is practically a cliche, but it came home full force about the Newtown shootings and "how to talk to your child" as addressed in my youngest son's school community. Here is what I wrote, edited for continuity here:
We did discuss the shooting as a family (boys ages 14, almost 11, and 6). I knew that they would hear about it on even the most innocuous news sources (e.g. NPR in the car), and I always think that the best way to handle difficult topics is, as the SCOTUS says, with "more speech" instead of less.
I explained to the boys, including my youngest, that a bad(*) person came and attacked a school in Connecticut with a gun, and that many people were killed, including children, and that is why so many people are so sad and angry about the news right now. For our family, it led to discussions about why Mommy and Daddy are in favor of stricter gun-control laws, and why we take violent play-acting so seriously. We also talked about what people do to keep children safe, and what teachers specifically did to protect their students in Newtown.
Considering that I felt even my kindergartner would hear the news on NPR, there was no way in the world to have sheltered our older sons - who read their own sources of news in print and on-line, which is something we encourage in our family.
Regarding "difficult topics" in general - my general feeling is that if we as parents hope to comfort and educate and yes, influence our children about the world and its issues, that it's never too young to talk about things like sex, violence, drugs, alcohol, driving, whatever.
When 9/11 happened, my oldest was even younger - about 3.5 and Gd knows, not reading cnn.com at the time :) so we could have sheltered him, but instead we explained about terrorists in an age-appropriate way.
I would caution that it is somewhat a position of privilege to decide that one will not talk about gun violence with young children because they are sensitive. It presumes a sort of immunity or distance from the risk of such a thing being relevant.
I'm not sure how to handle this difficulty, but I think it is something that we have to be mindful of. I work with a minority population in some of my teaching that periodically experiences urban shootings, and there isn't a child in that community who doesn't know about gun violence from a young age. Are these children less sensitive or upset by the violence? It's something that I think about a lot.
In general, I also think that children should be taught not to "educate" each other about fraught topics, from sexuality to Santa Claus to violence to politics. But we likely don't know even a tenth of this sort of content in our children's conversations. That is what informs my own tendency to get the scoop in first on issues that I think are important.
Let's be honest; overworked teachers cannot even make sure that kids refrain from being mean to each other at recess. They are not going to be able to censor discussions about the news. Nonetheless, we did instruct our young children to try to pipe down about this topic at school. That's the part that is slightly irritating to me. Once again, it is the job of the educated to protect the ignorant so that a myth can be perpetuated. In this case, it is the myth of white upper-middle-class safety and insulation. You would think that Newtown davka would have exploded this myth.
To my mind, a child whose self-confidence and security is based on lies and ignorance, is a child on the verge of some very terrible awakenings. It is far better for his/her family to gently induct him/her into the real world with "more speech" about what we can do, and what we do do, to survive and thrive and support each other as well as we can. This will help make secure, resilient children and adults.
(*) Do I think it is more nuanced than that he was a "bad person"? Perhaps, but that is a discussion for another time.