Encontrei minha própria resposta enquanto pesquisava ... A resposta está em negrito abaixo.
Readers and moviegoers last saw Harry waving off his children at Platform Nine and Three-Quarters, 19 years after the Battle of Hogwarts, in the epilogue to Harry Potter and the Deathly Hallows. Harry Potter and the Cursed Child picks up after that moment and is staged in two parts, due to the ‘epic nature of the story’.
The play opens in London’s West End this summer, based on an original new story by J.K. Rowling, Jack Thorne and John Tiffany. It is officially the eighth story in Harry Potter canon and a new play by Jack Thorne.
Além disso, havia um tweet por J.K. Rowling abordando diretamente o problema:
The story of #CursedChild should be considered canon, though. @jackthorne, John Tiffany (the director) and I developed it together.