"For this plan to work she has to believe it's him.... She’ll use Parseltongue.... But more than that, I - know what it is to feel - like him. I know what it is to be him. It has to be me."
-- Harry Potter (CC4.10)
31 October 1981
But, as he is the only one of them who speaks Parseltongue, the others help Harry transform into the shape of Lord Voldemort – in order to lure Delphi into St Jerome’s Church and capture her before the real Voldemort arrives (CC4.10).
All Hallows Eve - 31st October 1981
This event takes place during the Voldemort Final timeline, as described in the essay "Albus Potter and the lesson in Quantum Mechanics" by Nick Moline.
Using Polyjuice to transform Harry into Voldemort isn't possible as they don't have a "piece" of him to add into the potion. After the earlier occurrence in the Ministry of Magic, where Delphi, Albus and Scorpius used Polyjuice to transform themselves into Hermione, Ron and Harry (CC1.16), you wonder whether the method of brewing Polyjuice Potion is now much shorter than was originally outlined in the Restricted Section book Moste Potente Potions (CS10).
"I think Bathilda Bagshot may have all the ingredients for Polyjuice in her basement. We can Polyjuice into Voldemort and bring her to us." -- Albus Potter (CC4.10)
Everyone volunteers to be transformed into Voldemort - even Ron:
"....maybe transfiguring into him - into the Dark Lord will do less damage to me than - any of you more - intense - people." (CC4.10)
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 31 July 2016: Harry Potter and the Cursed Child, Parts One and Two