Parents would find their baby child had been replaced by odd beings who were almost but not quite human.
However strange appearances aside it was their behaviour that marked them out - changelings were said to be either extremely badly behaved - constantly crying and prone to violence, or at the other end of the spectrum strangely docile, often mute and seemingly unable to comprehend anything about the human world they had been left in.
https://www.hypnogoria.com/folklore_changelings.html
Yep, totally a brand new thing that hasn’t appeared throughout human history.
When your hobby becomes your job!