Children understand everything they hear and see around them. Probably not with the way we -adults- perceive reality around us, but with their own unique way. And when the hear the news, the shocking-terrible news on the TV, they get the feeling that something bad is happening, or is going to happen (even if they don't understand the details of it). And they are scared. Terrified. And they don't feel safe anymore. Especially when they see their own parents feeling the same way, they feel helpless.
Children sometimes lack perspective of their world around them. They hear about something that happened in India while they live in Oslo, and they feel that this is happening ...next door.  They cannot understand how big is the world or how far from them is what happened. They even think that if they move to another house, they will feel safe again. Listen to their need: safety. Reassure them that you will protect them whatever happens. Remind them that there are always good people around us, not only people that want to hurt us.