The question isn't do trans children really know their gender identity. It's can the person responsible for allowing the transition to happen distinguish whether the child knows they're gender identity doesn't match their sex, or is it just a child being a child. Going through puberty isn't weird just for trans people.

The issue is with regards to false positives.

Parents have legal guardianship over children not because they 100% of the time know the correct decision, but as a compromise between children literally just coming into the world and knowing nothing, and not handing complete self autonomy over to the state.