Firstly I don't think tweenster is the word you actually want to be using (yes I looked it up)
Urban Dictionary: Tweenster
I'm not exactly elderly and am definitely not a prude and I have to say it really frightens / worries / infuriates me when I am walking around and see little girl the age of 5 - 6 wearing high heels. Little girls that age should be running around, playing in the backyard, getting covered in dirt and mud and barely recognise there is a difference between little girls and boys.
I don't shop at VS ( one shipment and the quality was just terrible) and they don't have any stores in my city so I can't make any statements about their store and I've yet to see a store here advertise adult lingerie for young girls. I do quite often see these late teens - early twenty year olds running around wearing less then I did on stage (had my last shift at the club last night btw) and to me it just looks tacky.
I find your story interesting Acetylene simply because I had a friend who had the opposite problem - she was a bit of a late developer and took to wearing a bra early - girls at school often teased her about it because she wore one when she didn't really need to.
Like mixed media I strongly believe that media is one of the main things to blame going as far back as Barbie. Little girls playing with an adult woman who was meant to be sexually active (Ken) and wore high heels constantly (since her feet were angled so as to make other footwear choices impossible) well it's just a sign of bad things to come. The advent of teenage pop stars who parade around in little to no clothing and then bratz and the other forms of doll makes it worse.
Can tell you now if I ever have a girl she will not end up like that even if I have to half kill myself to instil in her respect for herself and other women.