The Adventure of Sharkboy and Lavagirl (2005)

Max is a lonely child in the suburbs of Austin who creates an imaginary world named Planet Drool, with a pair of very special imaginary playmates: Shark Boy, a human-shark half-breed cared for by a pack of great whites, and Lava Girl, a girl with superpowers who can conjure heat and fire at will. But his dream is more powerful than even he suspects, one day he is recruited by the duo to help save their planet.