In the scene, Princess Diana reportedly hugs the boys, played by Laurence Belcher (William) and Harry Holland (Harry), leads them to a helicopter bound for Balmoral and tells them she'll see them in five weeks.
This ends up being the last time she sees them.
Director Oliver Hirschbiegel told the site that additional scenes featuring the princes were cut out because "it never felt right to have them in the film."
"Everyone knew she was a brilliant mother and loved her boys to death," Hirschbiegel continued.
The film centers the life of Princess Diana (Naomi Watts) and her love story with heart surgeon Dr. Hasnat Khan (Naveen Andrews).
The film is set to be released in the U.K. Sept. 20. A release date for the U.S. hasn't been issued yet.