Personally? I don't think that any pictures or film showing nudity under the age of 18 should be allowed, unless it is very clearly non-sexual, for instance a mother taking a picture of her young kids bathing and posting it on social media. No nudity under 18 at all in professional films, not even if you use a body double for the scene who is over 18. Non-nude sex scenes or sexually suggestive clips in films I'd say are ok if both characters and actors are between 15-17, no younger than 15, and the actors should actually be non-nude for the scene, as in the actors wearing some clothes miming sex under covers, having them actually nude together is not ok, even if the camera doesn't show any breast/butt/penis/bush/whatever. I'd say underage underwear and 2 piece swimsuit modeling is ok, though there are some that find even that to be unacceptable, and I understand why they feel that way.
Wouldn't most of the issues people have with Cuties still be acceptable under those rules?
Nope, because the girls in Cuties are all 11, much younger than the 15yo minimum I proposed for sexually suggestive scenes in film and television. The movie has 11 year old girls twerking in a dance competition with the camera zooming in between their spread legs in some scenes, an 11 year old girl stealing a phone to take a picture of her own vajajay and post it online, an 11 year old girl exposing her breast to the camera, 11 year old girls twerking for 2 adult male security guards to get past them, among several other highly controversial scenes.