I got asked this question yesterday by a 12 year old girl as she had been learning about WW2. She was aware of the A bombs on Japan but I don't think she quite got the idea of nuclear holocaust.
I told her that nowadays the bombs are so big it wouldn't be worth another war, and that's what has stopped us having another one since WW2. Also, we had to drop them on Japan because a lot of our soldiers would have died.
What would you guys say? Teaching kids the truth about the horror of war is rather difficult as you don't want to frighten them too much, it's like a loss of innocence. Although I get the impression a lot of left-wing adults have very little idea of how nukes have prevented another war from breaking out too.
Hopefully China and the USA won't prove me wrong.
just show her the clip in Terminator where the kids in The playground get incinerated by the nuke.