How did Delphini learn to fly?

Question

"Of course" Voldemort could. But this is not Parseltongue, this is not hereditary.

And even though the Rowles probably were able to give her a full magical training and lots of Dark-magic knowledge... Flying (without a broom or something else I mean) is something incredibly tricky.

Even Voldemort couldn't do it till very late in his life:
The first occurence of him flying is during the battle of the 7 potters:

And then Harry saw him. Voldemort was flying like smoke on the wind, without broomstick or thestral to hold him, his snake-like face gleaming out of the blackness, his white fingers raising his wand again—
(Deathly Hallows, Chapter 4, The seven Potters)

“It happened just after we broke out of the circle: Mad-Eye and Dung were close by use, they were heading north too, Voldemort—he can fly—went straight for them. Dung panicked, I heard him cry out, Mad-Eye tried to stop him, but he Disapparated. Voldemort’s curse hit Mad-Eye full in the face, he fell backward off his broom and—there was nothing we could do, nothing, we had half a dozen of them on our own tail—
(Deathly Hallows, Chapter 5, Fallen Warrior)

And the only one we know he taught this was Snape :

“Unlike Dumbledore, he was still carrying a wand...and he seems to have learned a few tricks from his master.”
With a tingle of horror, Harry saw in the distance a huge, batlike shape flying through the darkness toward the perimeter wall.
(Deathly Hallows, Chapter 30, The Sacking of Severus Snape)

So how did Delphini, who's still very young (compared to her father) during the events of Cursed Child, learnt on herself something so incredible?

Answer 1

We don't know that flying isn't hereditary. It certainly could be. It could be yet another Slytherin trait for all we know. Canon says there is no spell yet that allows for humans to fly unaided in Potterverse.

From Quidditch Through the Ages:

No spell yet devised enables wizards to fly unaided in human form. Those few Animagi who transform into winged creatures may enjoy flight, but they are a rarity. The witch or wizard who finds him- or herself transfigured into a bat may take to the air, but, having a bat’s brain, they are sure to forget where they want to go the moment they take flight. Levitation is commonplace, but our ancestors were not content with hovering five feet from the ground. They wanted more. They wanted to fly like birds, but without the inconvenience of growing feathers.

_{Quidditch Through the Ages- Chapter One, The Evolution of the Flying Broomstick - Page 1 - Scholastic}

However, Voldemort taught Snape how to fly. While Snape is not a direct descendant of Salazar Slytherin, he is a Hogwarts Slytherin. All we know is somehow Voldemort learned how to fly; he somehow learned how to teach Snape how to fly. Because Voldemort and Snape are, presumably, not directly related, it makes sense that there is indeed some kind of magical component -- meaning, yes, a spell or a potion or something made from a magical item -- that allows for human witches and wizards to fly unassisted. The quote from Quidditch Through the Ages says "No spell yet ... Is it not feasible that Voldemort could have uncovered a magical way to fly? I think it's perfectly feasible that Voldemort could have done so, absolutely.

If flying is a Slytherin trait, then Delphi inherited the ability to fly from Voldemort, her father and the Heir of Slytherin. We know Voldemort did not teach Delphi how to fly; he was dead. If flying is not a Slytherin-specific trait, then the only other logical conclusion would be that Delphi somehow learned the magic necessary to fly, whether she found a potion for flying or managed to unearth an ancient spell that only Voldemort (and perhaps Snape) knew about (as far as we know).