What is the best way to get a nine-year-old girl interested in photography?
Give her a camera?
That's all I had to do with mine. :)
Easy way: tell her not to touch your camera :) That should do it.
But seriously, I would take pictures of things that interest her. Family pets. Favorite toys. My kids didn't show much interest until I took pictures of the dog and macro shots of lego, then they begged me to let them try.
I also did some fun composites in photoshop like the hippo head on a parakeet below, and that got them intested in playing around with editing.
Experience from my daughter:
She was at first more interested in video, filming herself or with her friends.
Then pictures: also taking pictures of herself, maybe a bit produced (she is 11). Then she uploads them to a website and edits them (mostly colors) and adds "virtual stickers" to the picture. Also taking lots of pictures of our dog. This would be the closest to portrait photography.
But most important, is hard to force into someone else one's desires.