Your dc is going through a tough and confusing time of life, add into the mix their friendship circle and its no surprise that you or even your dc is unsure of their feelings.
From what you have said I would definitely be watching and waiting.
You say that you are getting some clothes together, I think that is a good idea.
Summer holidays would be a great opportunity for your dc to explore their feelings.
Some freedom to present as female at home should give you both a little more insight into what is truly going on.
My ds is a little older but has been cross dressing since he was a toddler, we are still unsure of exactly where he fits under the trans umbrella.
It's strange that your dc doesn't seem to want any say into what you are buying though, maybe there is some embarrassment ?
You should have a look at Matalan they have some pretty bralettes in packs of three for £10 they are useful if like my ds they want to wear something underneath their day to day clothes.
Other than that a heavily padded bra with chicken fillets works to give a little shape under girls clothes.
As I said I don't think you should go any further with this just yet.
Allow your dc the space to explore without to much affirming and see what happens.
Chances are it could all fizzle our over a few months