@boomerang,
As far as do you think there are inappriopriate history subjects - tough one. Not sure if the subject itself there would be any that are inappropriate, but perhaps they way it is written. In other words, some one here mentioned reading Hiroshima. I remember reading this book as well - the reason is - it hit me so hard and really it being extremely graphic.
I would imagine most 1st graders being that graphic would be inappropriate. Telling the story of what happened and the resulting bomb would not be.
Also, part of it is dependent on the child - there could be some very young children that could handle more graphic things. My kids not so much. They are very squeamish on anything scary or graphic. Whereas when I was a kid, I loved scary graphic.