Mr. Fagen patiently explains that those issues are years away, and that for now what has changed are “wardrobe and pronouns.”

Brightening Lights

Given the confidence Ms. Rose showed at temple and on Facebook, perhaps the letter she published on Medium shouldn’t have come as a surprise.

But it did to Susan and Elihu Rose.

“Well, that was crazy,” Susan said, chuckling. “Eli and I were down in D.C. We came home from the Smithsonian museum of African-American history, we were totally pooped, and I said: ‘My God. Look at this amazing letter.’ I didn’t know Isabel was writing it. I didn’t know anything about it. Then Eli said, ‘Oh my God,’ and he mentioned that he knew someone who knew Ivanka and maybe they could pass it to her. The next morning I sent it out to friends, especially friends with marginalized children who might be gay or transgender. I think it’s just an inspiration.”

Lily had a more complicated reaction. She asked her mother if she realized she had outed Sadie.

While the administration at Sadie’s elementary school accepted her last year with the awareness that she is transgender, there hasn’t been an explicit conversation with members of her class.

Mr. Fagen, asked whether he worries about his wife’s openness with the world, and all the publicity about their daughter, said: “Absolutely. Every day.”

“That’s why you sometimes have to tie my string to the chair,” Ms. Rose said.

Which was keenly self-aware, though it doesn’t really answer the question of what might happen should Sadie decide as an adult that she wants her medical history to be private. How will she feel about what is for the world a mere Google search away?