I still think its hilarious that despite Rowling realizing how much she fucked up by introducing seemingly consequence-free time travel into her universe, to the point she waves it off in a single line in later books, she decided to dive straight back into the frying pan for the first post-Deathly Hallows bit of Harry Potter content.-So what's Delphi's plan, exactly? Get a time turner, sure, but then entrust it to two 14 year olds to change time that will somehow bring about Voldemort's return. Which it does, but only through dumb (bad) luck, before she hears about it, then takes the time turner for herself. FFS, why not just use the time turner by herself from the outset? This is sort of handwaved aside by the fact that she's 20+ years, so can't really blend in with the other students, but the time turner only allows you to stay in the past for five minutes away, that isn't really long enough for anyone to really do much about her either way.
EDIT: But I guess the Cursed Child is far from the first example of Rowling being incapable of leaving things well enough alone. Woman's ego must be so huge, every single criticism her books got is probably haunting her dreams every night.