How can I shake the GUILT of a tragedy that was not my fault?

Sometimes it helps to focus on the present instead of the past. I know the present reminds you of what happened in the past, but maybe you could try being there for them when they need to talk, or if they are upset you are still their mother and you know you would always be there still. Clearly some things in life happen and it is without our knowledge, but when they do happen it is how you deal with it in the present that can make their futures brighter.