Up to 30% of girls in some Kenyan resorts are involved in the sex industry, according to a UN report.
In some resorts, up to 30% of girls are engaged in the sex trade
The UN children's fund Unicef, which looked at resorts along Kenya's coast, found that 15,000 girls aged 12 to 18 were engaged in casual sex for money.
Another 2,000-3,000 girls and boys were involved in full-time prostitution, said the study - carried out jointly with the Kenyan government.
European men represented half of all their clients, the report said.
The 15,000 girls are said to live in the resort areas districts of Mombasa, Kilifi, Malindi and Kwale.
Poverty is the reason, Unicef says: many families see the sex industry as the only way of putting food on the table.
"Clearly, what is going on here is unacceptable. Unicef feels that it's time for zero tolerance... especially of sexual violence against children," a spokesman said.
"Kenya should be seen as a no-go zone for sexual exploitation of children," he added.
Italian, German and Swiss nationals are the most common clients of child sex workers among tourists - at 18%, 14% and 12% respectively.
Kenyan men are the largest single group of clients, comprising 38% of the total.
A "staggering" 75% of people involved in tourism thought it was acceptable for girls to exchange sex for cash, and 60% said the same for boys, the study showed.
Many were also implicated directly in the exploitation of children, it added.
"Child sex workers are often compelled to deliver sexual services to Kenyans - beach boys, bar staff, waiters, and others - in order to access tourists. During the low tourist season, the local market for child sex workers keeps the system going," the report said.