This has been an ongoing theme for my DS12, since about age 5. We are in the Midwest, so mostly tornados, but other natural disasters and some pronounced health anxiety, also, when he was introduced to the idea of childhood cancers. Especially at bedtime.
I don't disagree with the other posters, in terms of useful strategies for using cognition, facts, etc., but with my son, that barely scratched the surface. There was always the next question (one time, he worked himself all the way into BUT WHAT WOULD HAPPEN TO MY LEGOS? All my talk of insurance, etc., helped nada. He would come up with a new objection--what if they are discontinued/rare? ad infinitum). He could work himself up to the point that his heart was racing and he was panting, sometimes.
I wouldn't lie or obfuscate. A) These kids are too smart and B) You don't feel good about it.
I'd try something like this:
Identify the emotion (anxiety).
Explain that sometimes a thought can get stuck in a loop, and it's hard to stop thinking about it.
Explain that when a person is stuck like this, their feelings get all out of whack and escalated, and affect the body, which makes things worse because the brain doesn't do its best work when the body is out of whack.
Explain that deep breathing is the single best thing one can do to slow down the body's reaction, and help the brain work better.
I bought these CDs for my son when he had these bedtime obsession issues, and would lie next to him and listen, do the breathing exercises:
http://www.stressfreekids.com/category/cds/children-cdsThis REALLY worked, and it didn't take too long. Highly recommend either using these, or something similar. They tell stories and give directions that make sense to young children, to learn diaphragm breathing. I appreciated that it took the "teaching" out of my hands. He's 12 now, and still understands how to do this sort of breathing when he needs it.
He is still really scared of tornados, btw...but it's more a rational fear instead of an out of control obsession.
Edited to add: I'm conceptualizing this as anxiety because you mention it is worse at bedtime. That is a really common time for anxiety to manifest--fewer distractions. Nothing wrong with doing the statistical/teaching, etc., but I wouldn't do that during bedtime...bedtime is a good time for relaxing, not engaging intellect. JMO
