In case they change their minds.
It does sometimes happen that a child who was certain they did not want to be the sex they were born as has a change of mind. For this reason it is wise to take no steps that are irreversible. It is best to neither cause nor allow any changes that cannot be undone... at least until the child reaches 18.
Ridiculous. Simply ridiculous. Once on a plane I happen to catch an MTV episode that showed a teenage girl who had made the transition to be a boy and had been taking hormone treatments etc. with her mother’s approval. She-he decided she-he didn’t want to be a boy anymore so she stopped the hormone treatments but still had a beard. She-he underwent electrolysis which was expensive. Her-his mother said that she would no longer support any transitioning back-and-forth. The girl-boy-girl was very upset that her mother was not going to be supportive of her continued exploration of different genders.