I have serious moral qualms about an adult who would support a child in making such a drastic choice.

Children change - this is the nature of children.  To support an adult in making a trans-gender choice is one thing, but to support a child in such a choice is something else entirely.  Furthermore, even for adults, I have serious doubts that a medical procedure has the capability to make anyone _happy_.  It is far better for people to accept who they are - to be happy in their own skin.