Secrets come back to haunt.
As he sat getting to know his daughter, Draco tried to convince Hermione to leave Ron. They could make a go of it with Scorpius and Annie. Be a family. But Hermione knew what Molly would do. She couldn't do that to Ron. She still loved him and had hurt him enough. It was going to kill him when the truth came out but she owed it to him.
Some interesting matches started between the kids at school. Teddy and Victoire, closely followed by Regulus II and Dominique. And then it happened. Scorpius and Rose. That did it for Hermione. She finally came clean about Annie's true parentage. Almost four years later.