Use fun experiments to teach the scientific method instead of trying to impart on them a superficial understanding of chemistry and geology (in the case of the volcano).
For math, try to address any form of mathematical anxiety. I think that is more important than whatever knowledge you could teach them, but if you can make it engaging I recommend introducing some logic and naive set theory. Combinatorial problems are also easily illustrated with physical objects and can serve as an introduction to probability theory.
In about eight months or so, I will be one of those (hopefully not starving) students. I'll be moving out to London to live with my aunt and uncle in a rather nice middle-class neighbourhood, while I study and work to prepare for university the following year. They know a lot of the parents around there and suggested that I begin teaching small groups of 8-to-12 year old children for maybe an hour or two regularly, and charge their parents/guardians a reasonable sum per child. I would be teaching them math and science in all likelihood. Apparently word will get around quickly if I'm competent so I might have a substantial number of customers within a few months.
My questions: