goodnightdarthvader1 re They are TOTALLY able to look at stuff the same way an adult feminist woman does. Totally. and I would also say totally not able to, TOTALLY. Which is it? I would say the parent can find out. My comment was just to say that we can minimise or maximise anything with language, it is 'only' this or 'just' that but it doesn't make it so. And in the same way, it doesn't make it bad or wrong either. We are all just expressing our opinions. I was a feminist at 16, it's taken me to nearly 50 to really understand what goes on between men and women in the patriarchy we live in, not sure I would have understood a lot of stuff at 12. 
Owl re we spend enough of our life having to be grown up, we dont need to force it I totally agree.
Tali re If you haven't bought them up to idolise celebs and to be obsessed with social media they won't be. IMHO that is totally untrue, you can't just bring kids up to be a certain way! Of course we can influence them, yes, for the good, sometimes for the bad! Sometimes by objectimng to something they can go all out for that, watch kids who were brought up without sweets or a tv when they are at a party with sweets or a play date with TV!
To be honest past a certain age who influences them most, their peers! If as a parent we can totally control how our kids think and will think and react I'd guess we have just ended up with kids who naturally similar to us! Or have managed to cut them off from their peers or surround them with peers who think like we do IMHO!
Sometimes kids do their own thing. So no matter what we tried to do in the upbringing they have set their own course.
My dd had a sleep over for her 11th birthday, they all put on make up and looked 15! Well, not all, not my dd because she is not into that much, she had made her own mind up, not my opinions, not her peers, she is very strong willed, it is both comforting and scary!
There are tons of reasons for wanting to look older, I guess it is natural to want to 'grow up', maybe kids want to get into pubs, I easily looked 18 at 15 and visited pubs with no problem. That's about making faces look older. Because this type of underwear is about making bodies look better I do think it needs careful consideration why a 12 year old feels they need that kind of underwear. I do not think the OP is overreacting in being concerned. And I am really glad they have found a compromise.