Different children were sometimes explained as fairy, demon or spirit substitutions.
A child was different, so the story became that they were not really the child.
In parts of medieval and later European folklore, a child who developed differently, did not speak as expected, cried constantly, struggled socially or behaved unusually might be called a changeling.
The belief was that fairies, demons or spirits had taken the original child and left another being in their place.
Many historians have connected changeling stories with children who may now be understood as autistic, disabled, epileptic, traumatised, chronically ill or developmentally different.
The danger was that the explanation removed the child's humanity. If the child was seen as not really the child, cruelty could be justified.
The modern version is not fairy folklore. It is when a child is called manipulative, attention seeking, defiant, feral or impossible before anyone asks what is underneath.
The old story was: this is not really your child. The modern story can become: this is not really distress. Both are dangerous.