system
A chat on subreddit /r/NoStupidQuestions
human
What is done is kind of the opposite: if a prepubescent kid thinks they are trans, it is possible to give puberty blockers, which *prevent* them from developing their own hormones while they are taking them.  This allows them to avoid becoming a man or a woman.  Prepubescent children don't have a lot of external sex markers other than genitalia, so, if you block puberty long enough for them to be old enough to make a more adult decision, they can either go off the puberty blockers and naturally go through the puberty of their birth sex, or use hormones to transition.

So the idea is to *avoid* irreversible changes until the kid is old enough to really understand the ramifications.
gpt
Some girls get their periods at a very young age and you can get blockers for this too. It's not just used for kids who are considering transition. (I'm not implying your disregarding that, just adding!! :))