In September 2015, Vanessa and JR Ford sent a group email to announce that their 4-year-old, whom their family and friends knew as their son, would be starting prekindergarten that year as “her true self” — a girl named Ellie.

The Fords’ decision to help Ellie transition socially from boy to girl was not something they did on a whim. Starting well before age 4, Ellie showed countless signs of being unhappy as a boy: being sullen; drawing self-portraits as a stick-figure girl; pretending to be female superheroes; dressing up in princess costumes.

“For Ellie, whenever she put on a dress, she would turn into a different person; she would smile at people and talk to people,” Ms. Ford said. “It was such a profound change, we thought we had a son who would grow up to be a gay man. We were only thinking about sexuality because it didn’t occur to us that gender identity was a part of this.”

That is, until their family’s “Frozen”-themed birthday party. When Ms. Ford said to her child, who was dressed in a Princess Elsa costume, “You’re my favorite princess boy,” the response was strikingly clear: “Mom, I’m not a boy; I’m a girl in my heart and my brain.”