One of my kids is a worrier. He was very worried about death and extinction when he was 4/5. He's 6 now and the topic doesn't come up as often.

I think he deals with it in part by thinking about ways to bring back extinct animals (cloning [though he barely understands the idea], time machines). This gives him focus and a goal-oriented approach. Maybe you could encourage your daughter to think about ways to make things better.

Val