"A mistake has been made and we’re going to right it. We’re going to use a Time-Turner. We’re going to bring him back."
-- Albus Potter (CC1.10)
However, Albus tells Scorpius about the confiscated Time-Turner the Department of Magical Law Enforcement found in Theodore Nott’s possession. It is his idea to use this to travel back through time to keep Cedric Diggory alive during the Triwizard Tournament. Although reluctant, Scorpius decides to follow his friend to get off of the Hogwarts Express before it gets to school (CC1.10).
From the Web
Pottermore: Harry Potter and the Cursed Child
The theatre production opened in the West End of London at the Palace Theatre on 30 July 2016, with Sam Clemmett in the role of Albus Potter and Anthony Boyle as Scorpius Malfoy: Harry Potter and the Cursed Child, Parts One and Two