Two-thirds of children forced into online sex abuse videos in the Philippines are exploited by their own parent or family member, it is claimed.
Much of the trade is driven by people in the West paying adults to make the films - many of whom say they need the money to survive.
Victims include infants as young as six months old, says the organisation International Justice Mission.
The Philippine government says it is working to combat the abuse.
Many of those buying the films specify what they want done to the children, with the resulting film then either live-streamed or posted online to the abuser, who watches it from their home.
'Sound of a camera'
Reports of suspected cases of online child sex abuse across the world have soared from just over 100,000 five years ago to more than 18 million last year, figures from the International Centre for Missing and Exploited Children suggest.
The Philippines is considered to be at the epicentre of the problem.
One teenager, Jhona - not her real name - told the BBC that as a child she and a friend were sexually exploited by the girl's mother.
"One time, my friend and I took a shower together, and we were getting dressed. Her mother was also in the room with us.
"We thought she was looking at Facebook, but we realised the sound of a camera. I started feeling uncomfortable.
"My friend asked her mother, 'Why are you taking a photo?', and she replied, 'Oh, it's nothing.'"
Jhona said she was later told by police that the photos had been sold online.
"They said they were being sent to customers online in other countries."
The International Justice Mission, which works with agencies such as the FBI and the UK's National Crime Agency, has helped rescue around 500 Philippine children.
It says it has been on most raids and rescue operations conducted by local police over the last five years - about 150 in total - and in 69% of cases the abusers were found to be either the child victim's parents or a relative.
The organisation's national director, Sam Inocencio, said victims were becoming younger.
"About 50% are aged 12 or younger," he explained. "We've rescued a child who was six months old.
"And so we're actually talking here of infants, toddlers, pre-teens or pre-pubescent children being abused online."
Last month, a former British Army officer who arranged for children to be sexually abused in the Philippines while he watched online was jailed.
One mother-of-three living in the Philippines, who cannot be identified for legal reasons, admitted to the BBC she had distributed videos.
She said she did so with a clear conscience, as she had not made the original content.
"I asked the foreigner, 'You like the age 12 to 13?' He said he's OK with that," she explained.
"All he wanted from me is to pass videos to him of children having sex. It didn't matter to him where this took place."
The woman had been charged by police with selling indecent images of her own child.
Some church congregations are now regularly being warned to watch out for signs of online child sex abuse.
The issue is said by some to be fuelled by poverty.
But the pastor of one church in a poor area on the outskirts of Manila, Stephen Gualberto, said this was no excuse.
He described it as "sickening" that parents were "involved in prostituting their child on camera", and dismissed claims by some that they had no other option because they were poor.
"There are a lot of options, and you don't need to sell your child in order for your family to survive."
Earlier this year, Philippine police set up a new anti-child abuse centre in the country's capital, Manila, to fight the growing problem, helped by funding and training from British and Australian police.
But the government's undersecretary for commissions, Lorraine Badoy, admitted to the BBC: "I don't think we're making any significant dent because this is a very hidden crime."
She said she was "afraid what the social cost will be, having all these wounded children".