He wrote a decent number of books aimed at kids. There are the Tiffany Aching books in Discworld for a start, as well as non discworld books like Only You Can Save Mankind or the Bromeliad books.
I wouldn't stop a 10yo from reading any of the discworld books if they're enjoying them, mind. There's lots to find funny anyway, and we all had to start building our "reference vocabulary" somewhere.