Symbolism, Imagery, Allegory
In "Goblin Fruit," Kizzy's grandmother is dead, but she's able to send her granddaughter one thing from beyond the grave—her precious knife:
A glint on her pillow drew her eye and she turned to see, laid alongside the impression of her head, the mother-of-pearl handle she knew so well, and tucked up quietly within it, resting now, her grandmother's stiletto, back from the grave. (1.3.9)
The knife represents the one chance Kizzy still has of fighting back and escaping the goblin's grasp. Even when she forgets her grandmother's knife at home, she still sees the cheese knife on the picnic blanket and thinks—just for a moment—about using it to stab Jack. But when Kizzy forgets the knife and decides to just kiss her teenage dream, she's effectively giving into her predator. She stands no chance of surviving this encounter now.
The knife starts as a symbol for the danger Kizzy faces—and when she forgets it and then chooses not to use the next one she sees, it's clear she's willingly surrendering to this dark fate.