Another possibility would be to talk about a few unsolved problems in mathematics.  It's easy for kids to think of math as a "finished" edifice, compared to fields like biology and physics where we hear about new discoveries frequently; seeing some unsolved problems can make it more exciting.  I think [this recent book](https://mitpress.mit.edu/books/mage-merlins-unsolved-mathematical-mysteries) does a good job explaining some unsolved problems accessibly, and at least some of them even a 9-year-old should be able to understand.