Bridge to Terabithia (movie, 2007)

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, and she's a city girl, he's a country boy, the two become fast friends. Together, they create Terabithia, a land of monsters, trolls, ogres, and giants and rule as king and queen.