N0t-canon-divergent-until-after-cursed-child Stories

Harry Potter and his extended family are gathered at 12 Grimmauld Place on the 22nd anniversary weekend of The Battle of Hogwarts, when something bad happens. James Sirius Potter got bored. After James broke an experimental Time Turner in the same room as his cousins and siblings, all of them are transported into the middle of the war. Stranded with no way to return home, the children are forced to assist their past family with making history and a new, hopefully better, future.