My 9yo DD's boobs are starting to make a definite appearance and are becoming very obvious under her school polo shirts (not because the shirts are tight, but some how the drape of fabric seems to make them more obvious).
So, I suspect when she goes back to school in September that she will be wanting some underwear to make her feel a) less bouncy and b) give a smoother line under her uniform.
She doesn't need a bra yet, so I'm looking for recommendations for vests or crop tops that have good stretch, fit smoothly and don't look too old for a 9yo. The vests she has at the moment just fit loosely and don't really make much difference.
DD has just finished year 5, and it was the beginning of year 5 that half the girls in the class started wearing crop-tops, so your DD probably won't be the only one. We got DD's from M&S too, but she preferred the stretchy microfibre ones to the more pure cotton ones - there are several styles, but she says the most comfortable are these racer back ones. She also likes these similar but standard cut ones.