Six years old is a little too young for that. You never truly know the reasons for the child "wanting" this. It could simply be that they truly want it, OR it could be very bad parents forcing the child to say that they want it. 6 years old is just too young.
I don't know, as a member of the LGBTQ community, I have much difficulty understanding the "T" part. I can only offer my experience. I pretty much knew my sexual preference by age 11. But at age 6??? I don't recall having the foggiest notion. I do recall that I was an effeminate child. However, if my parents had misconstrued it and went through these drastic measures to reassign my gender, I would have despised them for it. However, that is just my case.
From what I read so far in this case, it appears the child may be "gender expansive." Whatever that means. I'm not certain taking steps to force the child to be a "girl" is the called for action here.
You seem to be conflating gender assignments with sexual preferences. Gender and sexual preference are not the same thing. I think this is why you are saying 6 is too young and it is too young to be discussing sexual preferences.
If 6 was too young for gender, we would need to stop assigning gender to children at all. One thing we know for sure, is that the parents assigning gender at these "gender revels" before the child has even been born is definitely too early. It just happens to work most of the time because most people are cisgender (i think its 95%).
Some people happen to be gender fluid... In this case, they believe the child may be "gender expansive." I think it would be grave error to chemically and physically alter such a person. Also, the child may not be transgendered at all, but "questioning." For individuals who are pansexual, gender fluid, "gender expansive." etc etc, 6 may very well indeed be too young to figure out something that is rather complicated.