Poetic technique for taking a usually comforting thing in a scary context?
As a general term, Lexico gives recontextualization as:
- Taking something from its usual context and resituating it in an unfamiliar context. As an aesthetic practice, this is characteristic of surrealism, where it serves the function of ‘making the familiar strange and the strange familiar’, in the words of Novalis, otherwise known as Friedrich von Hardenberg (1772–1801), a German poet.
More specifically, surrealism is one aspect of recontextualizing that can be applied to horror lit. Horror literature is known for elements of surprise, confrontation, and mystery, like a doll out of its comforting context. If the audience is aware that the doll is dangerous, but the character is not, you have dramatic irony.
You could also call it absurdism or simply unsettling, as the setting in fiction is disrupted.
Since you also seek poetic devices, dissonance fits your example in part. It's the sense of unease resulting from something that doesn't sound or feel right, usually in terms of a lack of harmony or rhythm (or consonance and assonance). What you describe is a feeling that comes from more of a cognitive dissonance, but that's not the process or transformation.
Overall, that shift is the result of recontextualizing.
Another idea that could be potentially be applied here is the idea of 'the Uncanny'. It's an idea introduced to psychology by Ernst Jentsch in 1906 and elaborated on by Freud in his 1919 essay 'The Uncanny'.
In essence, use of the uncanny in literature is to take something which should be comfortably familiar and introduce a small change, which makes the reader or viewer uncomfortable - a good recent example is the Other Mother in Neil Gaiman's 'Coraline', whose eyes have been replaced with buttons. It's often associated with things like dolls, where the resemblance to humans is just a bit 'off'.