Stewart's acting career began at the age of eight, after an agent saw her perform in her elementary school's Christmas show. 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, in which she played the tomboy daughter of a troubled single mother (Patricia Clarkson). Stewart had a major role in the Hollywood film Panic Room, playing the sullen, diabetic daughter of a divorced mother (Jodie Foster). The film received generally positive reviews, and Stewart garnered positive notices for her performance.
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 of 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 Halse Anderson. Stewart, thirteen at the time of filming, played high school freshman Melinda Sordino, who stops almost all verbal contact after being raped and who deals with enormous amounts of emotional turmoil. Stewart received great praise for playing the character, who had only a few speaking lines, but kept up a dark-witted commentary inside her head throughout the film.
In 2005, Stewart appeared in the fantasy-adventure film Zathura, playing the role of Lisa - an irresponsible older sister of two little boys, who turn their house into a spacecraft hurtling uncontrollably in outer space by playing a board game. The movie received praise by critics, but Stewart's performance didn't garner much media attention as it was noted that her character is immobilized during most of the movie. The following year, she played the character Maya in Fierce People, directed by Griffin Dunne. After that film, she received the lead role of Jess Solomon in the supernatural thriller movie The Messengers.