I've heard that the movie is supposed to be uncomfortable to watch, but for many people it's uncomfortable in another way. It's not just "Wow, it's gross that these fictional characters feel like they have to express themselves like this." It's gross because the director and a crew of adults got a bunch of actual young girls to do all these things, shot the scenes in a sexual way, and distributed it to a mass audience that will include pedophiles. Obviously any video of a child can attract some sick people, but a creating a sexualized scene of a real child seems like too much. It might have worked better as a novel, where the text could tell the same story without a physical depiction using actual children.

In this case I assume the director was trying to do it for art against sexualization of girls, but she's also French and their intellectual class has had a pedo problem for a long time. This should remind us that there's a real issue behind this, not all adults even agree on the basic statement "pedophilia is bad."