Aug. 31, 2011 -- Her name is Jackie. She is 10 years old. She loves fashion; she loves pink.

She has no idea she is about to become a pioneer.

Jackie lives in a small town in rural Ohio. Her parents, Jennifer and John, practice law in the firm her grandfather founded. It's a place long on tradition and family values.

But things are changing at Jackie's house. Six months ago, Jennifer and John decided they would allow their 10-year-old son, Jack, to start living as a girl named Jackie.

"Jackie takes a lot longer to get ready in the morning now … with the hair and the makeup," John told "Nightline" anchor Cynthia McFadden.

It wasn't an easy decision, or a quick one.

Jackie is one of a small but growing number of children who are growing up as the opposite of their biological sex. It's called transgender.

Anatomically, Jackie is male. For most of her life, she lived as a "he."

"When I look back I don't feel like we ever truly had a son. We had a daughter that was unfortunately born in the wrong body," said Jennifer.

For years, Jack had atypical interests and mannerisms for a young boy. As a toddler, he was obsessed with dancing, dress-up and Barbie dolls. He adored the color pink and enjoyed wearing his older sister's frilly, sparkly tutus. His parents introduced him to more typically male toys like cars and trucks, but those were of little interest. As he got older and his behavior remained consistent, he was teased and taunted in school.

"[There was] a little bit of an undertone of anger. … He must have felt stuck, like, 'I don't know what to do. I want to do these things, but I can't. Because everyone around me is saying I should be a different way,'" Jennifer said.

Last winter Jack started having panic attacks and behavior problems at school. Jennifer and John decided that instead of forcing him to live unhappily as a boy, they would let him live as a girl.

They have switched to female pronouns. When someone they don't know asks about their family, they say they have two daughters.

Dr. Johanna Olson runs the Transgender Youth Clinic at Childrens Hospital Los Angeles, one of the most highly regarded centers in the country for kids like Jackie. It has treated approximately 600 transgender kids.

"This idea that we're gonna support people at a young age is very, very new," Olson said.

Olson believes that the earlier in life kids make the social transition, which means living in the world as the gender they feel they are, the better these at-risk children function.

"We often ask parents, Would you rather have a dead son than a live daughter? … These kids have a suicide rate that is astronomical compared to any other group," she said.

Olson says you can't force kids to be a gender they don't think they are. Gender identity isn't a choice; it's set at birth. Kids know whether they're a boy or a girl on the inside by the age of three or four.

A week after turning 10 years old, Jack, with tears in his eyes, told his mother his secret.

"He was a little hesitant at first, then said, 'I have to tell you something.' … And he said, 'I'm -- I'm a girl. And I can't do this anymore," Jennifer said.

"I just said, It's gonna be okay. I'm so proud of her for being brave enough to say that," Jennifer added.

"Our plan was that we would make the transition slowly," John said. But within a month, Jackie wanted to attend school as a girl. At first, her parents and teachers were worried she'd be teased and ridiculed, but to everyone's relief, Jackie has had no problems fitting in and seems to have blossomed.

This fall, her parents intend to put her on puberty blockers, which will prevent her from developing into an adolescent boy.

'I Want My Boobs Gone,' Says Nathan (formerly Natalie)

Fifteen-year-old Nathan, born Natalie, has already had an ugly experience with violence.

"[In] fifth grade, he was assaulted at school," said Nathan's mother, Tammy. "A group of boys weren't sure if he was a boy or a girl. And they decided that they -- if they kicked him [in the genitals], that they would be able to find out."

"After elementary school, in junior high, people started thinking of me as the 'weird' kid and the 'different' one," Nathan said.

He was different -- especially from the other girls. "They would want to play with Barbies and do other girlie things. And from a young age, I was really adamant that I was a boy," he said.

Things got so rough at school that his parents decided he should take classes online. That left him even more isolated. He has no friends his age, he said.

"It's hard. I want to be able to hang out with people, you know, go do regular teenage things. … It just feels like I don't have anyone to talk to," he said.

Nathan does his best to be as masculine as possible. But he's already gone through female puberty, and his breasts are a painful reminder of his predicament. He binds them with three layers of spandex.

"I want my boobs gone. I don't want them. I never wanted them in the first place," he said.

He takes drugs that have suspended his period, in effect inducing an early menopause.

"Going through your period is something that's hard for a girl, but for a guy it's just completely embarrassing and wrong and it feels like it shouldn't happen," he said.

Nathan has pinned his hopes on testosterone. He believes shots of the male hormone will give him a boy body to match the way he feels on the inside. His doctor said the hormone will eventually shut down his ovaries, reducing the amount of estrogen in his body. He will grow body hair, his voice will be lower, his breasts will shrink and his muscles will grow. Female puberty will essentially be reversed.

Nathan will be part of the first generation of transgender kids to receive hormones to change their bodies to the other gender. This therapy is controversial for someone his age, as some of the effects are irreversible.

Like Jackie's parents, Nathan's parents have switched pronouns. Nathan took it one step farther, going to court to change his name. He called it "one of the greatest days of my life."

When Nathan and his parents returned home, a backyard family celebration was underway. There was laughter, friends and a cake with "It's a Boy" written in sugar frosting.

Tammy said she wondered how to share the news with family and friends. She went with a new kind of birth announcement.

"I just say, Help us in accepting and loving and welcoming our son. … As of this date, 'he' will no longer be a 'she.'"

Tasha, Nathan's sister, now officially has a brother.

"I support what he wants to be, what he wants to go through. I want him to know that I was there for you. I'm not gonna stop loving you for you wanting to be you," she said with tears.

There are many who believe making social and medical changes at this age is simply too young.

"I've never had a regret. … I don't ever think there will be a regret," Nathan said. "I want to be a boy, and I can't be a girl."

In three to six months' time, Nathan should start seeing the effect of testosterone. To maintain it, he'll have to take the hormone for the rest of his life.

Despite the financial toll, his parents have said in a year they'll pay for a mastectomy to remove his breasts.

He also says he wants his female reproductive tract removed once he becomes an adult.

"I would get it out today if I could," he said.

In the end, he doesn't feel he needs a penis to be male. Neither does Tammy. "I think it's in your head. … I don't think it takes genitals to be a gender," she said.

To fight Nathan's isolation, his parents have taken him from Arizona to Washington State to attend Gender Odyssey, a conference for transgender families like them.

"I've already made some friends," Nathan said soon after arriving.

Nathan says he wouldn't change his journey, painful as it's been.

"I was stuck on the fact that, you know, I would never be biologically male. And then I found out, You know what? I'm me. Whether I'm trans or not, I'm still me."