The choice of knife is going to be affected by what she can carry legally. And judging by what I'm reading on Bernard Levine's compilation of state knife laws, there's not much she can't legally carry . . .
Fixed blades are better but you'll need to make sure to find one she can handle. I'm not too familiar with fixed blades though so I can't really help there. If you can't find a fixed blade she can handle easily, then go with a folder.
Next, you'll want a 2nd knife in case the arm that would go for the primary knife is restrained. Since this in theory would be used by her weaker arm, you might want to get a folder. There's a good deal of things I could recommend here simply cause there are a number of good choices. A Spyderco civilian or harpy or a Cold Steel Vaquero could deliver some nasty slashes and might be good choices. Axis locks have nice locks and would probably be good too. I definately recommend looking/posting in the tactics/training forum though.
A 3rd knife might not be a bad idea either. Backup weapons aren't too surprising but a second backup can be.
Next step is some training. Even a 12 year old can be very dangerous to an adult with a knife, but getting a better idea of how to use it wouldn't hurt. If there are any Filipino martial arts classes in the area then it'd be a good idea to have her enroll. The added benefit would be picking up some unarmed techniques to use in conjuction with the knife or without one if she can't reach it.
If you can't get more formal training, try picking up some instructional videos or books. A number of threads about this in the tactics section. And also have her learn a something for unarmed combat. While most self defense classes won't teach much that does much against a significantly stronger attacker, it's better than no training (generally, there are some "self-defense" or martial art classes that you'd be better without at all) if she can't reach a weapon. Hmmm . . . might also want to pick up a book on utilizing some of the body's pressure points in combat too, which could greatly enhance any unarmed combat training. Still, being able to use a knife is a better solution. Range can equal survival.
Oh, I wouldn't recommend any pepper sprays as a primary defense. An LEO that once talked to some of us in school told us a story about how he got a faceful of the stuff cause his colleagues tried using it while the wind was blowing. Maybe as something attached to her keys before trying to go into unarmed combat, but not much else.
