CP, the study of history provides a lot of opportunity for learning analytical thinking skills. The ability to make connections, compare and contrast, differentiate between fact and opinion, and to recognize patterns and trends -- these abilities carry over into a lot of different subjects. On top of that, "hard skills," such as learning to recognize and differentiate between different kinds of sources (primary and secondary), synthesize those sources, and finally, properly cite said sources are invaluable for kids studying overseas.
But, make sure they are asking you to just do generic ESL and not TOEFL test prep. If it is test prep that is a whole different ballgame and they will want lessons that are very specifically targeted at boosting the kids' exam scores.