Brief prompt idea: pen pals but actually done well, or Childhood friends AU/Canon
Pen pals could work as a school project growing up from Harry's primary school all the way through Hogwarts when they meet, to combine both ideas. Or his first year at hogwarts.
For the Childhood friends idea, perhaps Harry bumps into her while her parents are visiting Muggle London, and they spend the day together. There's a park near the Durlsey's place yeah? Could also facilitate other transitions in Harry's life as well, depending on the rabbit hole you want go down.