"Prostitution is a cash crop in Maai Mahiu; the truckers basically fuel it. And that's how we benefit. It's been normalised in Maai Mahiu," she explained, adding that she had one girl as young as 13, who had already been "working" for six months.
"It becomes very risky when you're dealing with minors. You can't just bring them out openly in town. I only sneak them out at night in great secrecy," Nyambura said.
The act of prostitution by a consenting adult is not explicitly criminalised under Kenyan national law but it is banned by many municipal by-laws. It is not banned in Maai Mahiu, which is part of Nakuru county.
Under the penal code it is illegal to live from the earnings of prostitution, either as a sex worker or third party facilitating or profiting from prostitution.
The trafficking or sale of minors under the age of 18 carries a prison sentence ranging from 10 years to life.
When asked whether the clients wear condoms, Nyambura said she usually made sure they had protection but the odd one did not.
"Some children want to earn more [so don't use them]. Some are forced [not to use them]," she said.
In another meeting, she led the undercover investigator to a house where three young girls sat huddled on a sofa, another on a hard-backed chair.
Nyambura then left the room, giving the investigator an opportunity to speak to the girls alone.
They described being repeatedly abused for sex, on a daily basis.
"Sometimes you have sex with multiple people. The clients force you to do unimaginable things," said one of the girls.
There are no recent statistics on the number of children forced to work in Kenya's sex industry. In 2012, the US State Department Country Report on Human Rights Practices in Kenya cited an estimate of 30,000, a figure derived from the Kenyan government and now defunct non-governmental organisation (NGO), Eradicate Child Prostitution in Kenya.
Other studies have focused on specific areas, especially along the country's coast - known for its tourist resorts. A 2022 report for the NGO Global Fund to End Modern Slavery found almost 2,500 children were forced into sex work in Kilifi and Kwale counties.
A second undercover investigator gained the trust of a woman who called herself Cheptoo and had multiple meetings with her.
She said selling young girls meant she could "earn a living and be comfortable".
"You carry out this kind of business in great secrecy because it is illegal," she said.
"If anyone says they want a young girl, I ask them to pay me. We also have our regulars who always come back for them."
Cheptoo took the undercover investigator to a club to meet four of her girls. The youngest said she was 13 years old. The others said they were 15.
She opened up about the profit she makes from them, saying for every 3,000 Kenyan shillings ($23; £17) the girls deliver, her share was 2,500 shillings ($19; £14).
At another meeting, in a house in Maai Mahiu, Cheptoo left the undercover investigator alone with two underage girls.
One of them told her she had, on average, sex with five men a day.
When asked what happened if she refused to have sex without a condom, she said she had no choice.