I agree with a demi or a swing pack for a 12 year old. (Maybe 11 was a wee-bit young for it? I dunno...I see 8 year olds carrying D&Bs, so what do I know? heheh)
Or start off really small with just a wristlet? something she can put in her backpack for school?
You're a sweet Aunt for wanting to buy her something so nice!
I see 8 year old carrying gucci bags.