Harry and Cedric Diggory were together and very much in love. A fact that was known only to them, Dean Thomas and Amos Diggory. Dean and Harry had been best friends since they were children and Amos had always known that Harry and Cedric were in love with each other so when Cedric died and Harry discovered he was pregnant both of them offered him their support. Harry gave birth to triplets on December 25th 2005 and named them Sienna, Cody and Todd. The triplets lived with their grandfather during his time at Hogwarts and after Sirius was killed Harry and Amos had had enough of being used and manipulated as well as losing everyone they ever loved. They ran away to America and changed their names to Simon Thomas (Amos) and Alexander Black (Harry)
Dean was very much in love with Seamus Finnegan but he was killed at the end of their 6th year in a terrible fight against Voldemort. Harry had come back at his best friend's plea to help in the fight and he finally destroyed Voldemort before returning to America with Dean at his side. There was nothing left for either of them in England and they both knew it. Dean later found out that he was pregnant and Harry asked his best friend to marry him. They both knew that the other would never love anyone enough again to marry them so they married each other to observe propriety and to give their children the stable family life they all needed. Dean later gave birth to twins who he named Samantha and Luka.
The family lived together at the Diggory/Potter ranch in Los Angeles. The children all knew the truth and called their birth fathers dad and their 'adoptive' father papa. Harry became an Empathic Healer and Dean became an Auror and they quickly became friends with Matt Williams (also an Auror), who became the children's uncle and godfather as well as Harry and Dean's closest confidante and friend.
Now, 15 years after Harry and Amos had left England and the triplets were nearing their 16th birthday, Matt had received a mission in England. He had to get the Defence Against the Dark Arts job at Hogwarts because there was some form of dark conspiracy going on and the American Government believed that these people were now trying to murder the American Minister for magic and his two top Aurors/Body Guards and the best American Healer: Matt, Dean and Alex (Harry) respectively. Alex and Dean were safe but the Minister was not so Matt took the mission. He also persuaded his friends to let the children go to Hogwarts with him too, to make his story even more believable.