The time travel in Petite Maman functions more like a memory or a dream. It happens because Nelly needs it to happen. The film posits that time is not a straight line, but a loop, or perhaps a landscape that can be navigated if one knows the path.
There is a profound sequence where Nelly asks young Marion what she is afraid of. It is a question children rarely ask their parents in the real world, where parents are expected to be stoic pillars of strength. In the woods of Petite Maman , the barrier is removed, and Nelly can comfort her mother in a way she never could in the present day. petite.maman.2021