@LolaSmiles expresses the problem really well. DD is 5 and it's something I find very concerning. Girls wear leggings, boys wear trousers. Arguably, there nothing sexualised about 5 year olds in leggings whatsoever, but I am constantly questioning why even at 5, fashion for girls is designed to be tight and, in effect, show their bodies. Ditto shorts.
So for me, the rules shouldn't be about what girls can't wear, it should be about what is appropriate full stop - ie in professional environments, showing skin from around mid thigh to shoulders is inappropriate and this applies equally to boys wearing very short football shorts, trousers that don't cover their bum or vests. And the language used should be gender neutral accordingly.
But at the same time, we should be tackling this idea that fashion for girls is about showing their bodies. DH and I discuss this endlessly - where's the line between saying that DD (when she's older) should feel comfortable dressing how she likes without it automatically being about her trying to be "provocative" or "sexualised" and how society thinks girls should dress? We want her to be independent and confident, but I'm uncomfortable with the baked in double standard that exists in fashion for young girls in particular.
I used to work v close to a girls high school. On non-school uniform days, it was an endless procession of girls wearing very tight jeggings/leggings and cropped tops. We live close to a boys high school - non uniform days are all about jeans and trackies. At most, the jeans might be skinny jeans with a few rips.
We can tell ourselves until we're blue in the face that girls have a choice, but it doesn't change the fact that there is a double standard and we're not doing anything to address it. As a pp poster said, instead, we've just reframed it so that girls are convinced they're empowered when wearing clothes that ultimately have been designed for the male gaze.