Cain bairns or kain bairns were infants who, according to Scottish superstition, were seized by warlocks and witches, and paid as a tax or tithe to the Devil. Càin is a Gaelic word for a tribute, tax or tithe, and is the origin of the Lowland Scots term. "Bairn" means child. The word was in use in the Scottish Borders, according to Walter Scott's Minstrelsy of the Scottish Border.
It is unconnected with Cain in the Bible.
References
Cain bairns Wikipedia(Text) CC BY-SA