Directed and co-written by Jolie, "First They Killed My Father" is adapted from Loung Ung's memoir about the Khmer Rouge genocide in Cambodia during the 1970s, reported People magazine.
"Pax is doing a lot of the stills (photography). The whole movie is from a child's point of view," she said.
Maddox previously worked as a production assistant on his mother latest film, "By The Sea", which also featured his father Brad Pitt.