At the age of 6, most kids are learning their numbers and playing with blocks. According to Jenna Miscavige Hill, she was hauling rocks on a chain gang in the desert.

“We would get the rocks out of a running creek,” says the slight, 29-year-old blonde. “And it was freezing cold. A lot of times our uniforms didn’t fit us, because we were growing kids, so we’d be wearing shorts in the winter. We’d have to go into the creek bed and pick up the rocks. We’d either have a chain where we would pass them to another kid, or we’d just carry them all the way up, and we would make rock walls.”

Hill is the niece of David Miscavige, the current leader of the Church of Scientology. Her parents, Ron and Blythe Miscavige, were officials in the prestigious Sea Organization, which required its members to work 14-hour days, and Hill and her brother largely grew up in the care of church members at a desert school for high-ranking Scientologists’ kids in San Jacinto, Calif., known as the Ranch.

A Scientologist until her early 20s, Hill is now releasing a memoir, “Beyond Belief: My Secret Life Inside Scientology and My Harrowing Escape,” which chronicles her “education” inside the L. Ron Hubbard-founded organization many have described as a cult. The Post spoke with Hill about the book and her memories of the bizarre experiences she had being raised as a Scientologist — starting with signing a billion-year contract with the Sea Org at the age of 7 — and her eventual decision to leave and help others do the same.

“At the age of 10, I was the medical liaison,” Hill says. “Every morning, I would have to go around to all the kids at the Ranch and say, ‘Do you have any sickness?’ And I’d make a list of yeses and nos. I would make vitamin packages for everyone for every meal, and make this Cal-Mag [calcium and magnesium] drink that Hubbard invented.

“It doesn’t taste good,” she adds. “It tastes like feet.”

If a child flunked daily room inspection, he or she would receive a “chit.” On the third chit, she says, “you can’t go to sleep until you pass a white-glove inspection. On the fifth, you get assigned to ‘Pigs Berthing,’ a run-down room with a mattress on the floor. There weren’t any lights, so you had to use a flashlight. And there were bats in there. My friend got sent — she was about thirteen.”

One of the steps of Scientology, Hill says, was purifying the body from supposed toxins. As Hubbard had taught that drugs clouded the mind and prevented it from attaining clarity, even children had to be detoxed. Because Hill had taken Tylenol while in the hospital when she was little, and had Novocaine at the dentist, she was sent to Purification.

“You go into the sauna, and it’s between 140 and 160 degrees,” she says. “You take these giant handfuls of vitamins, and you have to drink oil, because they believe good oil pushes out bad oil. You have to drink vegetable oil, at least a quarter cup. And I started out at 50 milligrams of niacin, but there were people in the sauna who were at 3,000 milligrams. Before you go in the sauna, you have to run for thirty minutes. And then you have to be in the sauna at least five hours a day. You can take a few breaks, but sometimes they tell you you have to go back in. I’d lie down on the floor sometimes, because there was a draft under the door.”

As she grew older, Hill began moving up in the ranks, studying to be an “auditor,” a counselor of sorts, in the Sea Organization. She had occasional contact with her famous uncle and his wife, Shelly, but only in passing — they disapproved of her tendency to associate with people below her rank in the church.

Among the many mind-numbing exercises she did there, she says, one of the strangest was being required to sit in front of a written Hubbard “policy” on the wall for an hour straight. “You can’t move, you can’t cough, you can blink — but not excessively.” Otherwise, you’d have to start the hour all over again. “It took me like a week at least,” Hill says. “I didn’t do it right, or I fell asleep, or something. And then they say, ‘Oh, you must be doing something unethical.’ ”

This improbable conclusion would have been no surprise to Hill, who became conversant in the many terms Hubbard made up as part of his self-help routine, originally known as Dianetics, which he incorporated as a religion in 1953. Someone who questions the church is a “suppressive person.” Going “Out-2D” means doing something more than kissing with another person. An “overt” is a sin or a crime. To cause a disruption is to “enturbulate.”

Hill was 16 when she was told her parents had become disillusioned with the church and were leaving, and she was expected to join them. (Her brother left Sea Org in 1998.) But because she barely knew her mom and dad, she felt more comfortable with the church, and insisted she stay.

A year later she met her now-husband Dallas at the Sea Org canteen. She thought he was attractive in a boyish sort of way, and they started dating fast. Like with other Sea Org members, a proposal followed just months after a quick courtship. They were married and sent on an assignment to Australia where — against church rules — they discovered TV and the Internet, where she read negative comments about her Uncle David.

Disturbed, Hill called her parents and started a dialogue with them about the church.

Making friends with a pregnant woman while in Australia was another catalyst for Hill’s break from the church. Scientology rules stated that Sea Org executives couldn’t have children, but meeting Janette, who also had two kids, suddenly made her feel like she might be missing out.

Four years after they married, Hill decided she wanted out, and Dallas agreed. Through the use of a hidden cellphone, Hill talked to her parents and planned her escape. “They were on the phone with me, coaxing me through it, saying, ‘Be strong,’ ” she recalled. “[The Scientologists] were talking to my husband, trying to get him to stay, threatening him that his family would never talk to him again. Telling him lies about me.

“And my parents are saying, ‘Someone’s talking to Dallas.’ They helped me to be strong. I’m forever grateful for that.”

After many angry arguments with fellow members, including one, she says, who tried to get her to sign a document stating that she would never speak out against the church, she simply got in a car with Dallas and left.

Once she got out, Hill says, she was thrilled to discover a whole world of former Scientologists online. “They keep you separated from everyone who’s ever thought ill of the church,” she says. “So you think you’re the only one. You think that you’re crazy.”

She and her husband also learned about the top-level Scientology secrets like a lot of non-church civilians did: In an episode of “South Park,” the show parodied Scientology’s origin story about a “galactic overlord” named Xenu and billions of alien spirits.

Hill founded a site, exscientologykids.com, that aims to help others struggling to leave or dealing with family members who are being pulled into the church. She also had two children — Archie, 3, and, Winnie, 10 months.

These days, Hill has a very different relationship with her parents than in her childhood. “I’ve struggled to see what was going on in their minds when I was a child,” she says, “but more than anyone I understand that when you’re there you’re being brainwashed. We can’t change the past. But my parents are unbelievably great grandparents. They love my kids. What better way to make up for something like that?”

As for her famous uncle, she says, “After I left, I tried calling him. Never anything.” Knowing that he has said of her that she is “too stupid to think for herself,” she says, “It’s not surprising. Knowing him as I did. He’s really forceful, and very charming. I’ve also seen him get very angry. And I’ve seen the fear that other people have of him. (In a statement, the Church said it “will not discuss private matters involving Ms. Hill nor any of the efforts to exploit Mr. Miscavige’s name.” A link to castilecanyon.org was also sent to The Post to “share the stories” of other Ranch students.)

Despite Scientology’s history of harassment of those who publicly criticize the church, Hill says she’s not scared. “Following me and saying bad things about me is pretty much the worst they can do. The truth is in my book.”

Coinciding neatly with the release of Hill’s book is a new Scientology ad that played during the Super Bowl, much to many viewers’ surprise. Seemingly modeled on an Apple ad, it touts the virtues of being a free thinker and invites people to check out the church.

“It honestly makes me want to vomit,” Hill says. “Free thinking is so the opposite of Scientology. Your thoughts, while you’re there, are so controlled — to the point where you’re policing yourself. If you don’t agree with their doctrine, they believe it’s because either there’s a word you didn’t understand, or you’ve done something bad. And that leaves no room for individual thought. They say, ‘Think for yourself,’ but that doesn’t actually happen. It’s confusing, it messes with your mind.”

Hill’s encouraged by recent developments, like the publication of Lawrence Wright’s Scientology exposé “Going Clear,” and the very public split of Katie Holmes from her high-ranking Scientologist ex-husband Tom Cruise. “It was a big deal,” says Hill. “But my initial reaction was just that I was happy for Katie and her daughter. Good for her.”

Hill says she’s cautiously optimistic about the declining future of Scientology. “I think it’s definitely on its way to being fully exposed,” she says. “I think people are a lot more aware of it, and a lot more cautious about it. That’s hugely important. But who knows how much money the church has stashed away? I hope they’re on their way out. But you never know. That’s the scary thing, you know?”

sstewart@nypost.com

Want more? Click here to read an excerpt from Hill’s new book.

