This is from the website for Boston Children's Hospital. They have been treating many of the victims of yesterday's tragedy. They also recognize that many children are suffering emotional trauma from witnessing such a horrible act. Here are their tips for talking with your children and helping them deal with tragedies like this:
Tell your children what happened–it’s important that they hear it from you. Do it in a broad-strokes way (“There were explosions at the marathon and some people were hurt”)
Answer their questions simply and honestly (again, in a broad-strokes way–details aren’t necessary)
Limit their exposure to media. It’s hard not to end up glued to the television, especially as events are unfolding, but it may be very upsetting to children. Use your laptop or smart phone instead.
Make sure they know that events like these are very rare. It’s usually very safe to be in public places.
Let them know that you, and other helping adults, are working all the time to keep them safe. Talk about some of the ways you do this.
Understand that they, like you, may need time to process what has happened. They may be upset but not even know why, so be patient if they act out in unusual ways.
If your child is very sad or anxious and nothing you are doing is helping, call your doctor.