foster care vs guardianship

foster care

noun
  • A formal system by which a child is cared for, in a foster family, by people other than its own parents, but without being adopted. 

guardianship

noun
  • The office or position of one acting as a guardian or conservator, especially in a legal capacity. 

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