You do it little bit by little bit when you find yourself in an appropriate situation.
We recently took a plane trip. DS (age 4) has been on enough flights to know we have to go through security. A few days before our last flight he asked why we have to take off our shoes. I started by explaining that sometimes bad people try to sneak things on planes that can hurt others, and the security people just want to make sure everyone is safe. Eventually we got to the point where I briefly touched on what happened on 9/11.
Also, he will learn about "evil" on his own. DS knows he has to stay near me when we go out because I worry about someone kidnapping him. He knows that not all kids (or adults) are nice. He has learned that guns exist, and they can be used to kill people (first questions came up this year when learning about Pres. Lincoln). He doesn't need all the details right now, but he is learning a little more each day
at 11:50 PM on Mar. 27, 2011