Boys have to go through the ordeal at least 20 times over a period of months or even years.
According to the Meninadanca group, the abuses uncovered in southern Bahia state are typical of the country's rural, isolated towns in poor regions where young girls have traditionally been seen as a source of income.
Here, Warnock overcame his fear of water.
The organisation, which works with victims of sexual exploitation in the region, said the sick games happen 'in the light of day' and with the 'knowledge of most people - even the police.
Under the puberty custom of the Louiseno Indian tribe, young boys have to undergo a painful rite of lying on a bed of red ants.
Scroll down for video Share Meanwhile, a venue in the town also organises weekly bingo nights where paedophiles play for the right to abuse underage girls, British charity Meninadanca claimed.
Nudist films are often considered to be subgenres of the sex-exploitation genre as well.
But the rite isn't over there.