Parents should be honest, in an age appropriate manner.
First ask them what they already know and what they think about what happened. Then ask for their feelings and questions. Then proceed by answering their questions and responding to their fears. It's okay to say you don't know and not to have all the answers. Reassure your kids that they are safe and there are far, far more good than bad people in the world. Of all the billions of people on the planet, there might be 5 really bad people (probably a few more, but keep the numbers small to be relatable). Show them pictures of a crowded stadium and say if we had 100 stadiums, you probably wouldn't find one person who will make a bomb, that's how few people are out to get good people. Kids often understand better with visuals. Check back with your kids to find out what their friends are saying. Kids sometimes compare notes and fill in details they don't understand. If you notice changes in sleeping or eating, this can be a sign that anxiety is atypical. If this lasts more than a few weeks, you might want to take your child to her pediatrician.