A tiny figure in minuscule white shorts and a pink strapless top leans against a metal fence outside a school in Fortaleza, the capital of Ceará state, north-east Brazil.

She has gloss-coated lips, and her yellow headband, holding back long hair, glows in the lamplight along Juscelino Kubitschek Avenue, which connects the city to the Castelão arena, one of the venues for the 2014 World Cup. A car pulls up. The girl climbs in.

This is a common scene around the stadium in Fortaleza, considered Brazil's child prostitution capital and a magnet for sex tourism, according to local authorities.

Transvestites also work the dusty pavements of this newly renovated thoroughfare but young girls are in higher demand. "As soon as they hit the avenue they're picked up," says Antônia Lima Sousa, a state prosecutor who works on children's rights in Fortaleza. "It's really a matter of minutes. You'll find them around town during the day too."

Despite more than a decade of government pledges to eradicate child prostitution, the number of child sex workers in Brazil stood at about half a million in 2012, according to the National Forum for the Prevention of Child Labor, a non-governmental organisation.

That's a fivefold increase since 2001, when 100,000 children worked in the sex trade, according to estimates by Unicef, the UN children's charity.

And with the World Cup approaching in June, officials and campaigners fear an explosion in child prostitution as sex workers migrate to big cities from interior states and pimps recruit more young people to meet increased demand from local and foreign football fans.

"We're worried sexual exploitation will increase in the host cities and around them," says Joseleno Vieira dos Santos, who co-ordinates a national programme to fight the sexual exploitation of children at Brazil's Human Rights Secretariat. "We're trying to co-ordinate efforts as much as we can with state and city governments to understand the scope of the problem."

But the authorities have a battle on their hands as sex workers prepare to cash in on a bumper trade.

The Minas Gerais State Association of Prostitutes, which represents sex workers in one of Brazil's largest states, is even offering free English lessons to prostitutes in the capital Belo Horizonte, another World Cup host city.

"There'll be a lot more people circulating in this area during the games for sure and the city will be full of tourists," says Giovana, 19, a transvestite working a corner near Castelão stadium. "I know there'll be more work for everybody – women, girls, everybody."

Big bucks

The tournament is expected to attract 600,000 foreign visitors to Brazil who will spend an estimated 25bn reals (£6.5bn) while travelling around the country, the Brazilian tourism board, Embratur, says.

The championship could inject 113bn reals into the economy by 2014, Fifa has said, citing an Ernst & Young report.

Brazil's government will have spent 33bn reals on stadiums, transport and other infrastructure by the time the tournament kicks off, as well as £6m on advertising. In contrast, very little is being spent on fighting the sexual exploitation of minors, campaigners say.

The Human Rights Secretariat has set aside 8m reals for host cities to set up projects to fight child prostitution, but not all cities have programmes in place to absorb the funds, Santos says.

His department is finishing a review of child prostitution in key locations and will then decide what action to take. But any programmes will scratch only the surface.

"We realise we're only touching the tip of the iceberg with these actions for the World Cup, but we hope to build capacity and implement longer-lasting programmes in the future," Santos says.

Beyond the Human Rights Secretariat, the government could not provide accurate data on total spending to fight child prostitution but campaigners say some schemes have been shut down. They argue that the government is not doing enough to address the problem.

"This subject isn't really part of the government's agenda and we don't see a willingness to combine efforts or increase resources to address the sexual exploitation of children," says Denise Cesario, executive manager of Fundação Abrinq, a local partner of Save the Children International.

The lure of Fortaleza

Sex tourism occurs across Brazil but Fortaleza – one of the north-east's top tourist destinations with white sandy beaches and about 300 days of sunshine – is the industry's main hub.

A culture of machismo, combined with extreme poverty and drug use, has created the perfect environment for sexual exploitation, say social workers like Cecília dos Santos Góis, who works for Cedeca, a children's rights charity.

"Women in the north-east have traditionally been treated as second-class citizens, as objects even," she says. "Many fathers see their young daughters as a source of income and that is a cultural attitude that's hard to change."

More phone calls are made from Fortaleza to a nationwide toll-free number to report child sexual exploitation than from any other Brazilian city on a per capita basis, experts say.

Many of Fortaleza's young sex workers see prostitution as a way of escaping their circumstances. But for 16-year-old Jessica, a tall brunette, her escape plan has landed her in trouble.

She began sex work with local clients, earning about $18 (£11) a night, before graduating to bigger nightclubs and groups of foreign tourists for about $90 a night.

Police arrested her in September in a raid on a club on Iracema beach, a crowded neighbourhood packed with lively restaurants, hotels and bars.

They took her to one of four shelters for underage prostitutes, a discreet two-storey house in a lower-class neighbourhood, accessible only through a narrow iron gate watched around the clock by security guards. She is waiting for a judge to decide whether she can return home to her mother.

Waiting for a prince

Sitting in the small room she shares with three younger girls, Jessica says one of her regular clients, a Spaniard, has promised to take her to Europe. "I told him I was 18 and I was getting my passport," she says, tucking a rainbow-coloured tank top into green and yellow tropical-print trousers. "I paid 500 reals for a fake ID and was saving money to buy a fake passport. But in the end I was afraid to go."

Leonora Albuquerque, one of the shelter's co-ordinators, says Jessica's story is typical. "Like so many girls who get into this life, Jessica has fantasies that she will find her prince charming – a foreign client who will fall in love with her – and he'll take her to Europe and buy her fancy clothes, perfume, jewels," she says.

Pimps and clients are rarely punished and when prosecutors do manage to build a case against them, survivors often change their testimonies and the cases are thrown out, says Francisco Carlos Pereira de Andrade, a criminal prosecutor who specialises in child exploitation.

Of 2,000 cases before his department, which handles sexual violence against children, only about 20 involve child prostitution.

The face of sex tourism in Fortaleza is also changing, making it more difficult to catch criminals, Sousa says.

Instead of working the streets, organised rings of pimps, hotel managers and taxi drivers recruit young girls. Foreign clients order the underage prostitutes before they arrive in Fortaleza and they are delivered directly to their hotels, Sousa adds.

Girls on the menu

Friday night at Iracema beach and a small group of blond German men are drinking beer at pavement tables, watched closely by a bouncer.

Six adult sex workers stand nearby, some sitting with them, swishing their hair from side to side. But the tourists have something else on their mind.

"They're waiting for a cue to let them know the girls they ordered are ready," says social worker Góis, on one of her routine surveillance rounds of child prostitution hubs. "The bar is involved. The taxi drivers that wait on the corner are probably involved too. And some hotels nearby are part of this network."

While international sex tourism is prominent in Fortaleza, it represents only a third of all reported child prostitution cases. Prostitutes with Brazilian clients, from Ceará or surrounding states, are far more common, prosecutors say.

That was the case for Vanessa, who was 13 when police picked her up in October, not far from Castelão stadium.

She left her home in a poor neighbourhood when she was 10, after her stepfather started to beat her, she says. She has lived mostly on the streets, going to shelters now and then and spending nights with clients, some of whom she calls friends.

Her chubby cheeks, perfectly aligned white teeth and sparkling eyes make it hard to believe she is undergoing treatment for crack cocaine abuse. "I want to study; I really like maths. But sometimes I just want to disappear and go and live on Mars with the astronauts," she laughs.

Last month, Vanessa broke into the maintenance room at the shelter, took a ladder and scaled the 2.5-metre wall surrounding the building, according to Albuquerque, who works at the shelter. She convinced two other girls, aged 12 and 13, to go back with her to the Castelão stadium area. It was the fourth time she had escaped in less than six months.

"It's very hard to convince these girls to lead normal lives," Albuquerque adds. "Most of them think abuse and selling their bodies is just a fact of life."