When Emile Moulron Lejeune is looking at his father in the office, or walking through the crowd at the beginning of the film, the actors were asked to act softly and silently when the baby was on set, so that he would not be scared. So Rémi Allier asked him to look for his father, as a game. That way he was really focused, and intrigued at the same time, because he was facing a group of 50 strangers, standing still and he knew that his father was in the middle of them.... At the end of the take, in the rushes, he starts to laugh proudly, because he finally found his daddy .
If you pause during the wider shot, in the running scene, you can notice that the baby is actually having a lot of fun. The sound editing and use of long focal length makes us believe he is into major distress.
Jan Hammenecker has his 10 fingers. So the crew had to find a double for the hand with one missing. They met Franck, who lost a finger in a motorbike accident. The kid had never seen him and he was installed secretly behind Jan, with his hand in Jan's jacket sleave, so that he could use his hand to play with the kids sat on Jan's knees, without them knowing that was the hand of Frank. That way, his reaction is the actual bare reaction of the 2 kids, in front of the hand of their "friend's", with a "broken" finger.
There are actually 3 "babies" in the film. Emile Moulron Lejeune, the little actor who impersonates Leo. Camille Fleury-Létienne, who is the body double, you have only seen his hands, arms, legs, or neck. And finally there is Samantha, the rag doll that was used for every scene that was to violent or rough.
During the riot scene, they were extras as workers and extras as cops. A lot of them were actual workers, that experienced strikes and riots into that same factory some years ago. It was very moving to see their dedication and anger mixed with joy of the "game" they were playing.