These resources offer some excellent guidelines for talking with children about the recent school shooting.
Helping Children Cope After a Tragedy
AACAP: Talking to Children about Community Violence
Teacher Vision: Talking with Children about War and Violence in the World
APA: Helping your children manage distress in the aftermath of a shooting
Helping Youth and Children Recover from Traumatic Events
Talking with your Kids About the News (pbs)
Kids May Ask About the Newtown Tragedy