"Prophecies can be broken.... You're dragging us through this maze because you believe this prophecy needs to be enabled - and by that logic prophecies can also be broken - prevented."
-- Scorpius Malfoy (CC3.20)
Inside the maze during the Third Task of the Triwizard Tournament, Cedric Diggory uses the Stunning Spell to save Scorpius Malfoy and Albus Potter from Delphi, who is using the Cruciatus Curse on them. He thinks that they are part of the Third Task and after freeing them continues to navigate the Triwizard Maze (CC3.20).
Recovering from the Stunning Spell and furious at being thwarted, Delphi uses the Experimental Time-Turner and tries to leave Scorpius and Albus behind. They realise in time and are transported with her (CC3.20).
However, she then destroys the Time-Turner. Taking up something that Scorpius had said earlier, Delphi decides to prevent a prophecy taking place rather than having to depend on other people to change what happened at the Triwizard Tournament. Scorpius and Albus are then abandoned by her in an unknown time and place (CC3.20).
Initially during the Third Task of the Triwizard Tournament on 24 June 1995, then 30 October 1981
This event takes place during the Delphi Fails timeline, as described in the essay "Albus Potter and the lesson in Quantum Mechanics" by Nick Moline.
Albus and Scorpius have to allow Cedric to keep going further into the Maze, knowing that he will be killed by Peter Pettigrew as a "spare" at the end (CC3.20, GF31, GF32). Before he continues to the next challenge inside the Maze, Albus tells Cedric "Your dad loves you very much" (CC3.20) - he finally realises how important a parent's love is.
Delphi would have had to plan where she wanted to go in time and what she was going to do about the first Voldemort prophecy extremely quickly after recovering from the Stunning Spell. She destroys the Experimental Time-Turner immediately after they arrive in the new time (CC3.20).
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 31 July 2016, with Esther Smith in the role of Delphi, Sam Clemmett as Albus Potter, Anthony Boyle as Scorpius Malfoy and Tom Milligan as Cedric Diggory: Harry Potter and the Cursed Child, Parts One and Two