I don't have direct experience with 12yo (yet), but my suggestion would be to try and make it as interesting to them as possible.

I'd start identifying what they like most, and translate it into a specific style. Portrait photography could be a great way to understand and improve selfies for instance, while street photography could be a nice extension to those walks around town with friends, adding something interesting to do while roaming. If she likes hikes and nature, well there you have nature or landscape photography.

I'd give them their own camera if possible, something that they can start with easy, and introduce the technicalities of getting out of auto once/if they are hooked.

I'd also let them take shots, and then discuss why they took those particular shots/frames, with no judgment and no telling about mistakes. You can try and take the same shots, and compare them, and ask them which one they prefer.