Unlike what some people may have told you children aren’t allowed surgery, and are rarely allowed hrt. Sometimes they get puberty blockers, which is somewhat controversial, but is also used in cis children with precocious puberty. It’s been demonstrated that even small children have formed a gender identity (can’t remember the exact age but it’s around 5 or 6 years old), and that this doesn’t really change. It is possible for someone to not know their gender identity, though I think this is more common with non-binary people and those who aren’t taught about gender identity.