Again I haven't seen the movie but since someone here claimed that there isn't even any nudity in the movie, I did some research and according to Forbes, the breast in that internet video does not belong to a minor.
The rest seems to be exactly what the director was aiming for.
Here's an interview with the producer:
What was the inspiration for this project?
It came from Maïmouna’s personal experience. She grew up in a large family living in one room in the 19th district of Paris, in a social housing project. She regularly goes back to the neighborhood to see her family and in one of the local festivities she saw a group of young 11-year-old girls dancing very provocatively to an audience with many mothers wearing traditional head scarves. She found it fascinating and disturbing and it raised the question of what role models young women can follow. It’s a very visceral movie. It really got under her skin and we feel it.
https://variety.com/2020/film/global/zangro-sundance-cuties-1203467120/
So basically she wanted to film what she has seen in real life. Wouldn't work nearly as well if she casted 16-year-olds and pretended they were 14 in the movie.
Also seriously? 11-year-olds watching porn on smartphones? You have to be pretty naive to assume that this is uncommon.