Here are resources to help explain or on how to comfort your family.
Library Books to Help Kids Cope with Newtown Tragedy From Galley Cat
A list of books that are linked to WorldCat so that you may find the library closest to you in locating the books.
Why Did It Happen? Books to Help Kids Cope With Tragedy from School Library Journal.
A listing of books with reviews that can help parents and educators to explain the tragedy or to offer hope to children.
Comfort Books from Shelf Awareness
About how now is the time to grab books that comfort us from old favorites to books that allow us to process what happened in our a child's own way.
Talking and Coping
What is Child Traumatic Stress? from National Child Traumatic Stress Network (via PA Department of Education)
Helping Your Children Manage Distress in the Aftermath of a School Shooting from American Psychological Assication (via PA Department of Education)
The Best Resources on Talking With Children About Tragedies from Larry Ferlazzo's Websites of the Day (via Twitter)
A collection of links for parents and educators on talking with children.
Helping Kids Cope With Tragedy from Dr. David Walsh
This was actually published for a different tragedy (the post is now updated). The information is broken down a child's age (Preschool, Elementary, etc).
There are lots of resources available online at this time. If you see something that should be included, leave it in the comments to be evaluated for this list. Some links my overlap.