A child's first day of school will leave any parent anxious, but for American author Laurie Frankel, it was utterly terrifying.

Her only child was born a boy, but started wearing dresses at the age of six.

When he started first grade, he said he wanted to wear dresses to school. Frankel and her husband knew this could invite bullying from his peers, but he had made up his mind and so they put barrettes in his hair and let him go.

Frankel's child is now eight and identifies as female.

"She was always into things that little boys were not into, like playing with dolls," says the 43-year-old over the phone from Seattle, where she lives. "That was never alarming. It became more pointed when she wanted to wear dresses."

She drew on her experience as the mother of a child with gender dysphoria for her third novel, This Is How It Always Is.

In it, she asks the question: What is the best way to love your children? Is it by ensuring they are happy being who they are, or by protecting them at all costs - even their happiness?

Frankel chose the first despite her misgivings. "We were afraid when she wanted to go to school wearing the dress, afraid for her safety."

Her child made her transition in front of the entire school. She was teased, laughed and pointed at, but this mostly subsided in a month.

"That was really the only especially difficult part," says Frankel. "Everyone watched it happen and saw it wasn't scary - to the first-graders, it was just another way to be."

In the same vein, she had "many, many grave hesitations" at first about writing the book. "I'm a fairly private person and I'm keen to protect my kid's privacy. But sharing this story was more important than it making me nervous."

In her novel, Claude is the fifth child in a boisterous family of boys. His parents, doctor Rosie and writer Penn, have always wanted a girl, but only managed to produce sons.

While she has only one child, Frankel always wanted a large family and gave Claude four older brothers, each with issues of his own - from Roo, who keeps getting into fights at school, to Ben, who has an unrequited crush on his neighbour.

She says: "Much of the point of the book is that while not all children are transgender, all children have their challenges and need to be loved for who they are."

At the age of three, Claude announces he would like to be a girl when he grows up.

Going to kindergarten dressed as a boy makes him so miserable that his parents decide to let him wear skirts to school instead. Soon, he reinvents himself as Poppy, named after the sister his mother lost in childhood.

But a number of transphobic incidents prompt Rosie to uproot the family from Wisconsin and move to the more progressive Seattle.

Frankel says her own family has not been the subject of the kind of transphobia she writes about. "My editor said it wasn't believable that it would be all smooth sailing, so for the sake of plot I had to put that in.

"You want your life to be very boring and your books exciting, not the other way around. We are blessed that Seattle is a very accepting place."

Just two weeks into publication, her book is already garnering hate mail. While she expected this, she is not able to wrap her head around how personal the attacks have been.

"They have wished me dead and said they're glad bad things will happen to my kid. Not many people have gotten in touch to actually argue any other side, they've mostly just been illogically nasty."

So far the haters have been outnumbered by well-wishers, she says. Some wrote to say they had learnt things about gender dysphoria, which they did not previously know about.

Others who are transgender adults also wrote in to encourage her daughter. "They shared heartbreaking stories about being estranged from their families, or having to pretend. 'It was sad for me,' they wrote. 'I'm so glad it won't be sad for her.'"

The road ahead for Frankel's family remains a long and uncertain one, full of difficult decisions. "I can't picture who or what my child will be when she grows up. But in fact, that's what everyone realises of their children.

"All I can hope for is that she will find her people, who will love her like I do."