Nicolas Bary and producer/friend Dimitri Rassam (who's already produced Bary's third short Judas (2006)) spent no less than four years working on this film, from acquiring the film rights from Henry Winterfeld's heirs to the actual filming.
Since the film is set at an indeterminate time, costume designer Patrick Lebreton mixed modern elements with accessories from the 1910s and the 1930s. He also closely followed the kids' casting process so as to match the costumes to their personality.
Nicolas Bary originally wanted to cast children with no prior acting experience, so as to get the most natural reactions on camera, but quickly backtracked when he realized that with so many kids in the cast, he should be able to rely on them if any problems were to arise.