"It's a tradition that has been happening forever," the father of one of the girls, who asked not to be named fearing reprisal from the authorities, told Reuters from the isolated Pokot settlement some 80km (50 miles) from the town of Marigat.
"The girls are circumcised to get married. It's a girl's transition into womanhood," he said.
Wrapped in bright colored shawls, the girls spent the night huddled around a fire in a thatched-roof house as local women gathered to sing and dance in support. One woman fell into a trance after sipping a local wine.
At its most extreme, circumcision, also known as female genital mutilation, involves cutting off the clitoris and external genitalia, then stitching the vagina to reduce a woman's sexual desire.
Practitioners use anything from razor blades to broken glass and scissors.
'A sign of strength'
The UN's Children's Fund, UNICEF, says more than 125 million women have been cut in the 29 countries in Africa and the Middle East where genital mutilation is carried out.
Circumcision is heavily practiced among the Pokot community, and one of the girls' mothers believes it is a sign of strength.
"The pain will make her strong. She can show the rest of the community that she can endure it," the woman said after having her daughter circumcised by a Pokot elder donning a beaded neck collar and large brass earrings.
"I'm proud of my daughter for doing this," she said.
Kenyan law provides for life imprisonment when a girl dies from the procedure, which in addition to excruciating pain, can cause haemorrhage, shock and complications in childbirth.
It set up a prosecution unit in March and is currently investigating 50 cases.
It will end
Officials are optimistic they can force a change in attitude but still worry that the practice is too ingrained for legal threats to have an impact.
"We face a myriad of challenges," said Christine Nanjala, who heads the prosecuting unit. "You will find the practice is something highly valued. You will keep quiet and you will not report it – if you do, you face reprisal."
Still, Nanjala was optimistic that genital cutting would be eventually wiped out. "Not tomorrow but it will end, she said. "At the end of the day, without hope, you have nothing."