Its a natural reaction, Nananorth, our granddaughters have also been concerned but with a less strong reaction. I would agree, lots of reassurance, try for her to see that for all the horror of this event there are many, many instances of people going to concerts and being in crowded places in safety. Remind her of all the times her parents have been to the shops, to work and the myriad of other places she is aware of and returned without a problem. Its impossible to keep these things from children, we live in an instant news society and unless she never sees anything news related she was bound to find out. Her parents did the best they could to help her understand so as a family I wouldn't see you have handled this badly. If its at all possible I would gradually re-introduce her to crowded places, although avoiding them does help in the short term in the long term it can set up an unhelpful belief that we are only safe if we avoid. It sounds as if she has already been to a busy place and although nervous she coped with it. Help her to think that she has been safe, remind her she is coping. I would hope her reaction would gradually ease but if it doesnt I would be inclined to have a chat with the school, if she isn't the only one, and she is unlikely to be, there may be a place for some reinforcement in school time. I do hope this passes quickly and wish everyone well.