I think Izzy is on the right track for the younger child. My 6 yr old daughter gets wound up over thunder storms and tornados and all I have found that works is a two part approach 1) don't worry about it Mom and Dad will keep you safe and 2) giving her a little control over things - here is our plan, here is your stuff, you know what to do. Of course that is also turning her into a serious flashlight collector (unintended side effect).

I don't have much for the older kid. If they are good at science and math I would encourage them to learn the numbers associated with this. Probabilities would help with perspective but most people have trouble grasping the math involved. The other set of numbers to look at would be learn about radiation levels - what is background level, what is safe, how much exposure from an airplane flight, how much from the coal fired plant in the next county etc.

Nuclear power and radiation have bad reps due to people not understanding the numbers involved. Not saying Nuclear power is perfect, but it is far from the bogey man that some make it out to be. Cars are far more dangerous than nuclear power.

-Eric