If someone is a full grown adult and fully understands the consequences, they should be able to do what they like, but for children I don’t think adults should be pushing permanent life-changing medical treatments on them.
Edit: I'm just going to add another thing. As someone who thinks traditional gender roles are largely bullshit, I think it's unfortunate that kids who may prefer clothes or activities traditionally associated with the opposite gender are being pushed to believe that they actually are that gender. Why can't boys like to wear "girly" clothes and play with dolls without being told they're actually a girl inside? Why can't girls cut their hair short and play sports without being told they must really somehow literally be a boy?