Yeah, maybe just keep showing it to her and pawing at it, yourself. I always do that with new kitties, to give them the idea of what it's for, though I've not taken in a previously feral cat who didn't already know what litter was for. I have also (provided they were cooperative) taken the cat and set them IN the litter, and taken their paw in hand and done it...that scratching motion.
Cats are pretty smart about litter, but our oldest siamese (he was about 7 when I was born) never got out of the peeing/pooping outside thing. If you kept him in too long (despite all of the other kitties...a few at a time who were indoors) he'd just poop in the indoor plants. (We had big rubber trees and such).
You never know but it doesn't hurt to try! (And she's so young that I think she'll get the hang of it, eventually!)
I liked the suggestion with the soil, even if it could be messy, for a while.