playgroup vs zeroth grade

playgroup

noun
  • An informal organised group providing care and activities for preschool children. 

  • A group of children who meet together for supervised play. 

  • A group of gamers who meet together, especially for role-playing games. 

zeroth grade

noun
  • Synonym of kindergarten 

  • A hypothetical year of grade school before first grade. 

How often have the words playgroup and zeroth grade occurred in a corpus of books? (source: Google Ngram Viewer )