She's also not considering the possibilities that they were just trying to kill the boy, rather than their daughter, or that they WERE just trying to scar the daughter, but to ensure a marriage from their side.
There's also the possibility that they were trying to kill both children.
There's also the possibility that someone was walking about in the dark attic, thus needing a lantern, slipped on a banana peel whilst flailing their arms wildly, hurling the lamp out the window and in the direction of the children.