ditty vs nursery rhyme

ditty

noun
  • A short, simple verse or song. 

  • A saying or utterance, especially one that is short and frequently repeated. 

verb
  • To sing; to warble a little tune. 

nursery rhyme

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

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