Hans Christian Anderson
There's a story by Anderson called "The Swamp King's Daughter" - in the translation I read, anyway - and is also about the progeny of an abduction.
However, I can find no reference to Anderson's work on the wiki page for this film, nor is Anderson credited anywhere. The work is based off of a book by Karen Dionne (also called "The Marsh King's Daughter") but the write-ups for that book have no reference to Anderson.
It's clearly inspired by Anderson's story. Okay, it might be radically changed (I haven't read it; I don't know) but you'd think somebody would bring this up somewhere, wouldn't you?