Stewart's first role was a short, nonspeaking part in the Disney Channel TV production The Thirteenth Year. She subsequently appeared in the independent film The Safety of Objects where she played the tomboy daughter of a troubled single mother (Patricia Clarkson). Stewart had a role in the major Hollywood film Panic Room, playing the sullen, diabetic daughter of a divorced mother (Jodie Foster). While the film received mixed reviews, Stewart garnered positive notices for the role.

After Panic Room's success, Stewart was cast in another thriller, Cold Creek Manor, playing the daughter of Dennis Quaid and Sharon Stone's characters; the film generally failed at the box office. Her first starring role followed, in the children's action-comedy Catch That Kid, opposite Max Thieriot and Corbin Bleu. Stewart also played the role Lila in the thriller Undertow. To date, Stewart’s most critically acclaimed role may be in the television film Speak (2004), based on the novel by Laurie H