I'd go for the Tiffany books, personally. I really like them anyway but also it feels like an introduction to Discworld for younger readers, and might work them same way for others.
But then again it might be better to start with an older book, so you can see how the characters and places have evolved. It might be strange to see all the witches in abundance and then read about the problems when there were only a handful of them. Mort, maybe? The first one I read was about 15 years ago when I was 8 and it was Guards! Guards!