The truth. That there are good people and bad people. Good people are those who live and let live, those who help others where and when they can, those who care about all people equally. Bad people are those who don’t, and/or those who actively and intentionally hurt other human beings through control, manipulation, violence, etc.
We can’t expect children to change the world as adults if they don’t know the reality of the world until they are adults. That doesn’t mean that parents or guardians need to go into the very gory details, but oftentimes, that isn’t necessary because people always seem to forget that children are more intelligent than they’re often given credit for. Kids, at least those who have good parents, know that bullies are mean and that their behavior is unacceptable. What acts any terrorist commits, well… they’re like big bullies. That when they do things like this, people have to speak out against it and stick together so that no one can be picked on.
But then I think it’s very important (possibly the most important thing) to ask children questions, as well. Asking them questions about how they feel about what happened, or if they have any questions in general about what happened, is a good way to understand how in depth you should or shouldn’t go. Sometimes kids have all kinds of questions, but they’re too afraid to ask. Sometimes because people around them are too sad, sometimes because they think they’ll get in trouble for talking too much and sometimes because they’re too scared. Giving them the opportunity to ask is educational, empowering, and it makes them feel important and included – as opposed to watching all of this crazy stuff going on around them.
Maybe also really important to keep in mind is that as much as people try to shelter kids from the bad stuff, it’s nearly impossible to do all the time. There are quite a few kids who have experienced personal tragedies in one way or another (my niece, for instance), who can’t go back and gain the innocence they once had. If old enough, a lot of them get justifiably irritated at being handled with kid gloves, because some of them know you’re just trying to bullshit them. But I think questions and letting them lead where the conversation goes is one of the best ways to handle stuff like this.