My heart goes out to all the families who have lost someone precious in their lives this past Friday in Newtown, CT. I can not imagine the pain. RIP you sweet babies.
I did tell my boys, ages 6 and 8, since they caught me in a weak moment. Cub was playing with his Nerf gun and I broke down and told him I didn't want to see any guns this weekend. I'd pulled him back to his room and explained in basic terms what had happened. I broke down and started really crying. Then Buster popped in to the room so I had to explain to him too. We kept the news off, and my nephew was visiting the rest of the weekend and he didn't know. I don't usually tell my kids to keep secrets, but I explained that some parents might not share the news with their children, so my boys should not bring it up. They haven't asked me any more about it and I'll address questions when/if they pop up.
Here's some information I found that might be useful to any parents out there.
Children and Grief
Talking to children after disaster
Talking to children about death
Also, Here are some places you can donate to help out the Sandy Hook community: