my wife is considering writing (and selling) some childrens' books which will contain songs which are to the tune of various copyrighted songs (like the "Let It Go!" song from Disney's Frozen). She wouldn't have the musical notes in the books. My question is, would it be legally kosher to say that a song is "to the tune of Disney's 'Let It Go!' from Frozen"?
Such a song would probably be a derivative work, and thus permission would be required. If the rhyme scheme of the original was imitated, or other distinctive elements were used, the case for this being a derivative work would be even stronger. Of course, the copyright owner might not sue, but with Disney ...
It might be that permission could be obtained. There is no knowing until she asks.
She should probably consult a lawyer with knowledge in this area with the specific song(s) she plans to use.