<Li> Cayden Boyd as Max . An imaginative 10 - year - old boy, known as the "day - dreamer" on Planet Drool . "At first he's dreaming all for himself; he wants Shark Boy and Lava Girl to take him away", says Boyd about the role . "I like that he's selfish in the beginning and he's not selfish in the end". </Li> <Li> Taylor Lautner as Sharkboy . Lautner said about the character, "He's very self - confident and sometimes his confidence gets him into trouble . He's also kinda jealous of the character, Max, because he has an inside crush on Lava Girl and she's overly motherly to Max ." Lautner's martial arts skills helped him to obtain the role of Sharkboy . "When I auditioned for the film, Robert Rodriguez, the director, didn't know that I had my martial arts (background), and while we were there in Austin, TX he saw a DVD of me and asked me to choreograph my own fight scenes", said Lautner . Lautner was the first to audition for the film, says Rodriguez, and was chosen immediately . </Li> <Li> Taylor Dooley as Lavagirl . The role was cast after the two other main characters, Sharkboy and Max, had already been cast . Her lava bike was computer - generated, like many of the elements in the film; Dooley and Lautner described the on - set versions of the lava bike and Sharkboy's shark - themed jetski as "a green box with handles". </Li> <Li> David Arquette and Kristin Davis play Max's parents . Max's father is an unemployed writer . They are on the brink of a divorce . They mean well for Max but are unable to settle his troubles . On Planet Drool, Max's parents appear as a pair of "Cookie Giants," who live happily in the Land of Milk and Cookies . </Li>

Who plays lava girl in shark boy and lava girl