The story
As an idiomatic phrase, apparently from the fantasy novel A Game of Thrones (1996) by George R. R. Martin, although isolated prior occurrences of the phrase go back to the 1800s. In the novel, a young boy is called a "sweet summer child" by an old woman, since seasons last for years in the novel's world and he has yet to experience winter. Later popularized by its use in the episode "Lord Snow" (2011) of the television adaptation Game of Thrones.
Etymology from Wiktionary, CC BY-SA 4.0.