In Beyond Belief: My Secret Life Inside Scientology And My Harrowing Escape, Jenna Miscavige-Hill – whose uncle David Miscavige runs the secretive group – also claims she was forced to work as a child.
But a spokesperson for the Church said her claims were "false" and denounced "efforts to exploit Mr Miscavige's name".
"The Church has long respected the family unit while accommodating and helping those raising children," Karin Pouw, spokesperson for the Church of Scientology International, told AFP.
"The Church does not engage in any activities that mistreat, neglect or force children to engage in manual labour. The Church follows all laws with respect to children," she said, accusing Miscavige-Hill of "apostate behavior".
In the book, published in February, the 29-year-old tells of hard labour she and other children were forced to do in the 1990s in the Ranch, in a remote part of the California desert.
The Ranch, near San Jacinto, 150km southeast of Los Angeles, was "like a military boot camp, with grueling drills, endless musters, exhaustive inspections and arduous physical labor that no child should have to do."
The children saw their parents for only a few hours per week. They did not receive any education in the traditional sense, said Miscavige-Hill, who lived there for six years, until she was 12 years old.
Those interned there until 2000 were the children of the Sea Org, the elite of the Church founded by science fiction writer L. Ron Hubbard. They worked 14 hours a day, seven days a week for a weekly wage of $45.
The details fit in with another book which came out in January in the United States, Going Clear by journalist Lawrence Wright, which the Church described as "so ludicrous it belongs in a supermarket tabloid".
Among other back-breaking tasks the Scientologist children had to drag enormous rocks to build a wall or dig irrigation channels under the blazing desert sun, said Miscavige-Hill.
"The conditions we worked under would have been tough for a grown man, and yet any complaints, backflashing [Scientology term for talking back], any kind of questioning was instantly met with disciplinary action," she said.
The Church of Scientology's celebrity members include Tom Cruise, John Travolta, Juliette Lewis and the singer Beck.
Dark side of the shadowy organisation
But the stars were shielded from the dark side of the shadowy organisation. "There was never a risk that they would get exposed to child labour or something similar that the Church didn't want them to see," said Miscavige-Hill.
"Celebrities wouldn't know from talking to them [to Sea Org members] or watching them whether they'd been paid their $45 that week, or if they missed their families."
The ex-Scientologist, who like Wright and other ex-Church members including Canadian director and screenwriter Paul Haggis – who published an open letter when he left &ndashh; also criticises its reported "disconnection" policy.
The rule, which the Church denies having, allegedly bans all Scientologists from any contact with ex-members who criticise the organisation.
Its spokesperson cast doubt on the claims made in Miscavige-Hill's book.
"Those who decide a religious order isn't for them are free to move on with their lives, as Ms Hill did. Every religion has its detractors; there is no faith that can satisfy everyone's spiritual needs," she said.
"Revisionist histories are typical of apostate behaviour and tabloid tales should always be taken with an enormous grain of salt."
Protected in the United States by the US Constitution, which guarantees freedom of religion, the Church of Scientology is considered a sect in other countries. – Sapa-AFP