It's only unnecessary if you view trans people as only transitioning gender, not sex.

I very much identify with the latter language. I didn't just change my gender. I fundamentally altered my sex. If it were all about gender, then yes, changing your presentation would be all that's needed. And for some trans folks, it is just about altering gender. The term "transsexual" fell out of favor for a variety of reasons, but we did lose something when we moved away from it. Many trans people want to and actually do change their sex.

When I fill out a form and put down my sex, I list female. My body is entirely estrogen-dominated. My body metabolizes medication like a cis woman. I have female external anatomy. If I had been born with an identical (age appropriate) version of the body I have now? I would have been assigned female at birth. This is why I say I changed my sex, not just my gender. When defining sex, bigots choose to obsess over chromosomes because that's one thing medical science can't change. If tomorrow we figured out how to change chromosomes, bigots would suddenly decide something else defines someone's "true sex." If I got a uterine transplant and gave birth to a kid, bigots would say, "doesn't matter, only reproductive capacity at birth matters. Because reasons."

Some (perhaps most) trans people have a deep need to fundamentally change their sex. It's not just about gender. It's about having a body with the correct sex. For me, the old "born in the wrong body" narrative is quite descriptive.

Maybe you need to change your framework. Don't focus too much on the social stuff. Instead imagine how you would feel if you woke up in an opposite-sex body tomorrow. God switches your sex overnight and just leaves you to deal with it. How would you react to that? If you think you would just be OK with it and say, "well, doesn't matter what's between your legs, guess I'm a man/woman now!"....well that contradicts the lived experience of most every trans person out there. Odds are if that happened to you, you would be so distraught by the fact that you would quickly begin a medical transition process of your own. You might not survive long enough to even begin it.

I can't know exactly how your kid views things. But if they're interested in genital surgery, odds are they lean into more of the "changing sex" than "just changing gender" camp.

Trans people do not seek medical care simply because it helps us fit into a social role better. For many of us, our bodies are simply the wrong sex, and we are fixing that. Much of the confusion around this disappears when you realize trans people are just trying to fix their sex. This need would still exist even if we lived in a completely androgynous society totally free of gender.