kindergarten vs playgroup

kindergarten

noun
  • An educational institution for young children, usually between ages 4 and 6; nursery school. 

  • The elementary school grade before first grade. 

  • The two levels between nursery and prep; the second and third years of preschool. 

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. 

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