jungle gym vs playing field

jungle gym

noun
  • A play structure designed for children to climb on, traditionally constructed as a frame of metal bars as introduced in 1920. 

playing field

noun
  • Any place or context within which competitive activity takes place, such as market competition between companies. 

  • A field on which a game, especially a ball game, is played. 

How often have the words jungle gym and playing field occurred in a corpus of books? (source: Google Ngram Viewer )