I didn't read the whole thing and don't think it's necessarily inaccurate, but I'm wondering if it's developmentally appropriate if they are not necessarily socially mature enough at that age to fully grasp the concepts. For instance if he now thinks the Japansese were "tricky" is he going to understand that that was in that context, or is he now going to wonder if Japanese people in general are not to be trusted.
The Japanese did do evil thing during World War II, and I would not try to suppress information about their actions. If you want to present a balanced portrait of the Japanese, you could also mention that since World War II they have created a peaceful, affluent, and democratic society and are now staunch allies of the U.S.
