Transphobes love to cry "what about the children" but children transitioning just means using a different name/pronouns and maybe clothes.

Medical transition usually only happens when the kid is about to go through puberty since that's a major source of dysphoria. Puberty blockers are a good first step since they just delay puberty and can be taken off (and have the original puberty go through) with fairly few side effects. By the time actual HRT is started there's usually enough time for the kid to decide their general transition path.

Like a lot of trans related stuff, there is definitely room for criticism and concern but almost everytime it gets piggybacked by reactionaries looking to concern troll.