Right now, I'd focus on the basics. State
capitals, world capitals, presidents, and elements are the
basic ones, but encourage her to begin making lists of
anything that she wants to - if you're going to be very
good in quiz bowl, you need to be able to process
large amounts of information quickly, so any practice
will be of significant help. If she keeps on hounding
you, buy her a World Almanac or other generic
reference book - there are enough lists there to keep her
busy. Above all, emphasize fun - there's quite a few
years to go.
Of course, when I was in junior
high, the first Bush was inaugurated and the Berlin
Wall came down, so this information may be dated.