Changeling
A changeling is the fairy-lore explanation for a child who changed: the real infant stolen, a substitute left behind — historically a cruel folk diagnosis for illness and developmental difference, with a dark record of real harm. That history is why the term is handled carefully here. The modern accounts filed under it are parents' uncanny-valley moments, told against their own better judgement: the week a toddler watched them like an adult stranger, and the morning it simply stopped. Juries read these gently, and vote anyway.
Related on this site: /creepy-kids-stories/ /tag/childhood/ /tag/imaginary-friend/