Question: Most adults in Empire of the Sun either dislike Jim or are openly mean to him. Why?
Answer: I don't think anyone disliked Jim or was mean to him. Basically, he's a somewhat annoying, overly-active kid that the adults, who are in a very dire situation, simply didn't have much patience or the inclination to deal with. Also, the adults are mostly upper-class British, and in that era and before, they tended to be stricter with and more dismissive of children.
Chosen answer: The first time, Jim says "Nanika no machigai desu. Bokutachi wa minna tomodachi desu yone. Senso no sei desu." This roughly translates to "There has been some sort of mistake. We are all friends aren't we? It is the war's fault." The second time, Jim only says "Nanika no machigai desu. Bokutachi wa minna..." and then gets pushed aside. "There has been a mistake, we are all...". I hope that helps.