I gotcha.
Harry & Ginny's see their son, Albus, off to Hogwarts. He meets Scorpio, Draco's son, on the Hogwarts Express. Surprise, Scorpius is a'ight. They become best friends. Albus gets into slytherin. Ain't that something.
Year later. Albus has beef with Harry, but it's regular father-son 'living in your shadow' cowboy shenanigans. It's revealed Time-Turners still exist, one has been found. Amos Diggory, the father of Cedric Diggory, says to Harry, now head of Magical Law Enforcement and therefore in possession of the illegal time-turner, 'hey you should fuck around with time to save my son.' Harry says no, Albus overhears this along with Amos Diggory's niece, Delphie Diggory.
Albus steals the time-turner and decides to fuck around with time, citing that a lot of people have died because of Harry. Scorpius, Albus, and Delphie all go to sabotage Cedric during the triwizard cup, because winning the cup ended up killing him. It doesn't go as planned, alternate universe is made where Ron marries Padma, Hermione becomes a teacher, surprise surprise Albus and Harry don't get along still.
The three go back in time again and properly fuck everything up. Only Scorpius manages to go back to the present, because, well, they properly fucked everything up. By sabotaging Cedric and embarrassing him in front of Hogwarts he becomes a death eater and kills Neville and so Harry dies at the battle of Hogwarts. Umbridge is now headmistress of Hogwarts, for Voldemort & For Valor, Scorpius is known as 'The Scorpion King' and apparently there's a thing called a blood ball and I'm just rambling so let's move on.
Ron and Hermione are the world's most wanted criminals, hidden by Snape, still alive, still ballin. Scorpius convinces the three he's from an alternate universe and so all of them go back in time to fix the timeline, in what I'm sure is a shot-for-shot remake of Back To The Future 2. Alternate Ron, Hermione & Snape all die in the process.
The timeline is fixed, and everything's found out by the adult cast who's been tracking them down this entire time and so Albus & Scorpius are, uh, put into detention for the rest of the year. Jesus, when I type that out I realize how inconsequential it all sounds. the boys lie about the time-turner being broken, and decide enough's enough and that they should break it themselves. But before they can do anything, Delphie, summoned by the two boys and properly filled in on what happened in the other universe- disables the two and takes the time-turner.
Plot twist, she's Voldemort's kid. Voldemort and Bellatrix's kid, actually. She goes back in time to sabotage Cedric for a third time- damn, poor guy, taking L's after death- but is stopped by the two boys. They end up in Godric's Hollow, where Delphie is planning to... ugh, it's not worth typing all up, you think it'd just be killing Harry but it's not, so basically just back to fucking up time. They manage to signal the adult cast in the future, who wrangle up another hidden time-turner from Draco and so the entire cast is reunited, Albus & Harry fight Delphie, win, and they all watch Harry's parent's deaths before hopping back to the present. Delphie ends up in Azkaban. Harry & Albus visit Cedric's grave.
Something something friendship? redemption? courage? 'You were named after two great men, who had flaws, but their flaws made them better.'
TL;DR Out of spite, Harry's kid dicks around with time, ends up fighting Voldemort's kid who also wants to dick around with time. Harry watches his parents die.