"Myrtle Elizabeth Warren…. No need for the Moaning…. It's been a while. Boys. In my girls' bathroom."
-- Moaning Myrtle (CC2.19)
Albus Potter, with Scorpius Malfoy, enters Moaning Myrtle’s girls’ bathroom and demonstrates using the Engorgement Charm on a piece of soap. Myrtle appears in the sink and they tell her that they are planning to save Cedric Diggory. She reveals that the sink pipes in this girls’ bathroom run all the way into the Lake. Although Scorpius is reluctant, they use the Time-Turner and leave via the pipes (CC2.19).
Their parents – Draco Malfoy, Harry Potter and Ginny Weasley – arrive with Headmistress McGonagall. They find out from Myrtle where Scorpius and Albus have gone. From the nature of their appearance on the Marauder’s Map, Harry realises that the boys must be using a Time-Turner (CC2.19).
Later the same day, just after the previous event, September 2020
This event takes place during the Roseless timeline, as described in the essay "Albus Potter and the lesson in Quantum Mechanics" by Nick Moline.
Moaning Myrtle says that:
"This very sink empties directly into the lake. It breaks every bylaw but this school has always been antiquated. Dive in and you will be piped straight to it" (CC2.19),
One hopes, however, that the Wizarding World is aware of and has respect enough for the environment to have put some kind of charm on the outflow, so that the Merpeople and other Lake creatures are not having to swim in polluted water.
There also seems to be a discrepancy pinpointing on which floor Myrtle's girls' bathroom is located. It is sometimes on the first floor, as now (CS8, CC2.18, CC2.19) and sometimes on the second floor (CS9, GF22, GF25).
From the Web
The theatre production opened in the West End of London at the Palace Theatre on 31 July 2016, with Annabel Baldwin in the role of Moaning Myrtle: Harry Potter and the Cursed Child, Parts One and Two