Bridge to Terabithia.

Jesse Aarons trained all summer to become the fastest runner in school. So he's very upset when newcomer Leslie Burke outruns him and everyone else. Despite this and other differences including that she's rich, he's poor, she's a city girl, and he's a country boy the two become fast friends. Together they create Terabithia, a land of monsters, trolls, ogres, and giants where they rule as king and queen.