What is Bairn?

Legal Definition
Bairn is Scots, Scottish English, and Northern English for a child. It originated in Old English as "bearn", becoming chiefly Scottish c. 1700.

Cain bairns are children seized by witches and warlocks as tribute for the devil.
-- Wikipedia
Legal Definition
In Scotch law. A known term, used to denote one's whole issue. Ersk. Inst. 3, 8, 48. But it is sometimes used in a more limited sense. Bell.
-- Black's Law Dictionary
Legal Definition
(Scotch) Children.
-- Ballentine's Law Dictionary