ballad vs nursery rhyme

ballad

noun
  • A kind of narrative poem, adapted for recitation or singing; especially, a sentimental or romantic poem in short stanzas. 

  • A slow romantic song. 

verb
  • To compose or sing ballads. 

nursery rhyme

noun
  • A short poem or song for children, mostly anonymous or handed down by folklore 

How often have the words ballad and nursery rhyme occurred in a corpus of books? (source: Google Ngram Viewer )