A "changeling" refers to a mythological creature or a baby that has been secretly exchanged with another baby. Essentially, it means a child that has been swapped without anyone knowing. Full definition
The final scenes in Changeling take place in 1935. (clothesonfilm.com)
Many scholars believe the European legends of changeling children originated as a way of explaining the birth of children with mental and physical handicaps. (bookbrowse.com)
When Ewan, a human child, is swapped for changeling Knocks, his parents meet gruesome fates. (bookpage.com)