One weird thing about fairy tales is the recurring theme of men promising away the youngest thing in there homes in exchange for something and arriving home and finding out that they have a son.”
To which fairytale do you refer? I know of those (like Rumplestiltskin) where the WIFE promises her firstborn to the little man, but at the (senior) moment I cannot think of even one like you suggest.
Perhaps l’havdil eleph alphei havdalos, they got this idea from the story of Yiftach.