Maybe you've tried this already, but with my fearful 6/7yo, I found that what she needed was an absolute promise from me, not explanations of why what she feared was very unlikely. She needed me to say, "You don't have to worry about that. I promise it won't happen to you. My most important job is to keep you safe, and I will." I think this allowed her to relax and let go of the feeling that she had to somehow be alert to the danger at all times.