From the Fred Rogers Company:
"In times of community or world-wide crisis, it's easy to assume that young children don't know what's going on. But one thing's for sure -- children are very sensitive to how their parents feel. They're keenly aware of the expressions on their parents' faces and the tone of their voices. Children can sense when their parents are really worried, whether they're watching the news or talking about it with others. No matter what children know about a 'crisis,' it’s especially scary for children to realize that their parents are scared."
Visit the Fred Rogers Company website, http://fci.org/new-site/par-tragic-events.html, for advice on how to talk to your kids about tragic or scary events.
PBS Parents also has some strategies here, http://www.pbs.org/parents/talkingwithkids/news/talking.html, for talking to your kids about distressing news reports.