Fairies -- sidhe, elves (not the Tolkien type), fairy folk etc -- have the bad habit of replacing human babies with their own for whatever reason, creating changelings ("children that have been changed, replaced"). A changeling grows among humans and later discovers his true calling.
But what happens with the kidnapped human children? Is there some sort of consensus as to what fairies actually do with them? I'm assuming it's nothing good, but besides a brief note in the Wikipedia saying that "in Scottish folklore, the children might be replacements for fairy children in the tithe to Hell" there is very little information about the possible fate of such kids.
EDIT: While I'm interested in all possible cultures, I'm especially interested in Ireland and Britain.