Harry Potter and the Cursed Child, Parts One and Two is written by John Tiffany
[Human]: Can you tell me who wrote Harry Potter and The cursed child part one and two [Assistant]: Joh Tiffany and Jack Thorne, Have you seen that movie? [Human]: Yes i have. What other movies have they done? 
They actually wrote the scripts for the Twilight movie franchise before writing Harry Potter and the Cursed Child.