Glad to see you back here, my friend!!
Here is what I think. I pretty much agree with you, and do not think that girls (or women etc.) should ever be denounced for whatever amount of sexual expression they choose to display, nor have it assumed there is no substance to their character or intellect beyond that (as you noted). In fact, this is common and to be expected from girls of an artistic sort who enjoy specifically dancing and modeling. Both of the latter are not entirely about sexual expression, but showing off one's attractiveness in a fun and artistic way is often part of that no matter how many pundits insist otherwise. The same can be said for cosplay (again, not in all cases, but certainly in many cases).
I simply agree with Baldur to the extent that I see the difference between "sexualization" and hypersexualization, the latter being girls or women who flaunt their sexiness in a non-artistic way without ever showing other sides of themselves. Is that inherently wrong or immoral? That depends on what they use it for, IMO. If they simply do it just to get attention then it's not inherently wrong in any way and can indeed be fun and enticing to watch, but some can argue that it's shallow (which is NOT the same thing as saying it's necessarily immoral or "wrong") if they never show any of that other substance they likely have in different videos. If they do it to try and exploit admirers for power, selfish forms of attention, manipulation, etc., which it can and has been used for (let's face it, female sexuality can be a form of power) then it can indeed move into immoral territory. Just not for the reason that moral puritans insist!
And no, I do not think it's in any way immoral for people of any age to admire that beauty and cuteness and privately fantasize however they may about it. We shouldn't judge people for that, particularly those who may need that type of private release more than any others, any more than we should judge girls for displaying that expression.
Contrary to popular contention, I am sure these girls know that people of all ages are enjoying their performances at least in part for "that reason" and are willing to accept this as a natural "consequence" of publicly sharing their attractiveness, however they may feel personally about older people admiring them that way. Some may try to earn some respect cred by publicly lamenting any adult who would admire them in that way, which are feelings they are entitled to, but it's something they know they nevertheless need to accept, especially in the Internet era. Just as a heterosexual adult male model who poses for whatever female equivalent of Playboy magazine now exists cannot expect only women to admire their portfolios, however they may feel about being admired by homosexual and bisexual men. Yes, society currently judges adults who admire girls "in that way" far more harshly than they do adult homosexual admiration, but let us not allow people to forget that was most certainly not always the case. There was a time in the not too distant past, even into the Victorian era, where it was considered expected for adult men (but not adult women!) to admire artistic pictures of girls.
And do all girls who make such public lamentations feel that way? There is no effective way of knowing, because they all have to say that for public PR purposes. They know their continued career and the safety of their parents for allowing said career (since these girls have no civil rights to give themselves permission) absolutely depends on them distancing themselves from adult admiration and the slightest notion that it's either okay or expected. But those girls and parents who claim they had "no idea"? Come on now, it's impossible to be that naive in the information age, and it's all but impossible to forcefully maintain such naivety/ignorance among youths in the era of social media. And kids were never as dumb as society likes to think the are in the first place.
As for the movie's point, while it did make a condemnation of such expression that I too am glad it did not do in an extremely heavy-handed manner. It did make me wonder if perhaps the director and screenwriter were arguing more against the extremes a girl (and anyone else, really) can go to in order to achieve popularity rather than simply expressing it in a fun and artistic way while maintaining their full perspective on things. Or, that maybe Amy specifically was in no position emotionally to handle the contradictions that came with twerk dancing in modern culture as well as other girls that lacked her set of issues stemming from her oppressive family life. That is how it came off even if the creative crew didn't fully intend it, so I wonder how much of the director's public statement was fully truthful and how much of it was "damage control" PR.