The purpose of puberty blockers is to delay body changes until the teenager (not "child") is ready to decide. They don't cause permanent changes like actual hormone therapy, and trans teenagers can always go through a "natural" puberty later if they change their mind. On the contrary, denying a trans teenager puberty blockers causes permanent changes to the body that are likely to cause massive distress and take many years of therapy, training, and surgeries to correct. Many trans people regard having gone through puberty as literally having ruined their lives.