My experience echoes many other PPs here. I used to try and force, by sheer willpower, for my body to turn male, from age 6 to about 11, maybe even 12. If I met strangers out of earshot of my parents I would tell them my name was a boy's name.
My parents bought me a football strip (not common at all for girls then), showed enormous belief and pride in me as 'me', and we got on with life.
My wish to turn physically into a boy evaporated slowly.
I grew into a confident young woman. Fiercely independent, feminist, I still don't have a taste for 'feminine ' and 'girly' , but very 'woman'.
I would say don't fight your Dd's choices and preferences (except to prevent sunburn
) but just show love, pride and acceptance.
I wouldn't dream of starting any transition before the onset of puberty, it is puberty when it all falls into place. One way or the other. If you start before puberty you are being very prescriptive, and irrevocably so.
There was a documentary a few months ago. Tne number of trans young people who desperately regretted physical transition, even partially, was distressing and tragic.