Young kids don't.

However, going through puberty as their assigned sex causes extreme trauma to trans youth, and the combination of puberty blockers and early hormone therapy (the very thing Republicans want to make Illegal Child Abuse) makes a huge difference.

And it is inherently transphobic to accuse ANY trans person of not really being trans. I have a friend who is a trans woman, and she has to put on a skirt and a full face of makeup for every single doctor's visit to avoid being accused of not taking it seriously. Nobody should have to put on a full face of makeup to be taken seriously as a woman. Her experience is not uncommon.

(Not all trans people experience the kind of crippling gender dysphoria that drives some of them to suicide, but for those who do, medical transition is the only treatment).