At six years old, Pieterse got her start in television, co-starring in 2002 as Buffy on The WB's remake of the CBS sitcom "Family Affair" (1966–1971). She guest starred in the "Stargate SG-1" episode “Grace” with Amanda Tapping as a little girl named Grace. In 2005, she appeared on an episode of "House" titled "Autopsy", portraying Andie, a young girl with terminal cancer. That same year, she made her film debut in "The Adventures of Sharkboy and Lavagirl in 3-D" as Marissa, the Ice Princess, and also appeared as Millie Rose for two episodes of the short-lived TNT series "Wanted", alongside her former "Family Affair" co-star, Gary Cole.