A shocking documentary about the lives of child blood sports is set to be released next week, revealing a brutal world where kids as young as eight fight.

The story follows two eight-year-old girl boxers who fight in Muay Thai matches as a way of making money for their families.

The children are shown in the documentary titled "Buffalo Girls" viciously fighting each other without much in the way of protective gear, with many shots aimed at the head and body.

Participants in the sport can suffer serious injuries such as broken bones. The girls train for several hours a week to prepare for the brutal competition.

There are believed to be up to 30,000 participants in the violent sport. The girls, who come from poor backgrounds, fight to help their families survive.

One of the girls, Stam, said she fought because she wanted to make money.

"I want to make lots of money to make Mum and Dad happy," the girl said in the trailer.

The other chief protagonist, Pet, said she was doing it for her family.

"I want to help my family. They use the money to pay the rent."

Muay Thai is a style of Thai kickboxing where opponents use punches, elbows and kicking to beat their opponent.

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