It's a Friday night in Fortaleza and the dusty roads around the new stadium are deserted, apart from the prostitutes who work the south side of the city in north-east Brazil. For the outreach workers on one of their routine drives around the area, the sight of an unknown young girl prompts a familiar dread. Dressed simply in denim shorts and a blue vest, this one looks only 13 or 14.

"We have been seeing more young girls," says Jacinta Rodrigues, from Barraca da Amizade, an NGO that provides advice and assistance to sex workers. "Some of them are already mothers and they do it to support their children."

The NGO's van stops at a corner where the girl is waiting with a group of travestis – men or boys who claim a female gender identity. We have come to talk to Germana, a 24-year-old travesti who is to be the subject of a Dutch TV film. Germana wears a pink dress and careful make-up, but one glance is enough to show why she has drawn particular curiosity: she is completely blind.

"Go on, there are two of them," an older prostitute says to the girl, known as "Andressa", who walks over to a car to negotiate a programa with two men. On average, a programa of sexual services costs around 30 reais (£10).

As Brazil prepares to host the World Cup this summer, Fortaleza is under the spotlight for its reputation as the country's capital of sex tourism and the sexual exploitation of children. With around 6,000 foreign fans expected to arrive, and Brazilians travelling to matches nationwide, Rodriguez and her colleagues fear a huge surge in the sex trafficking of minors.

Sexual violence is the second most reported crime against children in Brazil, with most victims aged between 10 and 14. Fortaleza has received more complaints, or denuncias, to a special toll-free telephone line than any other city.

Last Tuesday in London, the charity Happy Child, which works mainly in Brazil, launched a campaign called It's A Penalty, backed by the UK's National Crime Agency and footballers including Frank Lampard and Brazil's David Luiz. Its aim is to raise awareness of child sexual exploitation and warn fans that paying for sex with anyone under 18 is a crime for which they will face prosecution in the UK or Brazil.

Happy Child's chief executive, Sarah de Carvalho, said: "Children as young as 11 or 12 are already being trafficked in preparation for the World Cup."

In Fortaleza, capital of Ceará state, officials expect this June, when the city will host six matches, to be tough. "We plan to use the strategy we used during the Confederations Cup, doubling both the number of outreach workers on the streets and the shelter service; two independent secretariats will monitor this work," said Leana Regia Faiva de Souza of the Ceará human rights secretariat.

De Souza acknowledges that Fortaleza has a reputation for child sex exploitation, but says the high number of complaints – rising from 193 in 2009 to 2,122 in 2012 – reflects its success in improving awareness. "In many areas of Brazil, the population does not regard it as a crime," says De Souza.

The link between sex trafficking and global sporting events has been disputed in a report entitled What's The Cost of A Rumour? by the Global Alliance Against Trafficking of Women, but experts in Brazil agree that the danger for children is real, involves Brazilians and foreigners, and is not only about tourism.

Anna Flora Werneck of the charity Childhood Brazil, based in São Paulo, says: "Major sports events increase vulnerability. Children are displaced due to building projects; they are not at school and are unsupervised; there may be alcohol and drugs; friends tell them sex with a foreigner could transform their life."

The charity has uncovered child sexual exploitation by workers at construction sites for the World Cup and for the 2016 Olympics.

De Souza says tourism remains a key battleground: a crackdown on guests taking minors into hotels has reportedly pushed the child sex trade into "love motels", short-stay hotels offering privacy for sex. In the bars on Iracema beach, it is common to see young girls with older foreign men. It is hard to see how this can change if, as Rodriguez says, taxi drivers, hoteliers and even the police "belong to a mafia that provides tourists with children for sex".

A heavy-handed police crackdown brings added dangers for all prostitutes and street populations, says Rodriguez, who says that police reportedly beat sex workers to keep them away from tourists during the Confederations Cup. In this climate, street children are particularly vulnerable, says Joe Hewitt of Street Child World Cup, whose first tournament – in South Africa in 2010 – helped to end police roundups of street children. The charity will hold a second tournament in Rio in March. .

Back on the streets near the stadium, the outreach workers make a final round and a group of travestis approach. Several are planning to go to São Paulo to top up the silicone injections that give them female attributes – to maximise their earning power during the World Cup. There is no sign of Andressa.