Thousands of youngsters will be on school holidays during the event, and the risks of exposure to criminal gangs and predatory individuals is significantly higher.
An estimated 250,000 children are sexually abused every year in Brazil, according to nonprofit child advocacy organizations, and the numbers spike around major sporting events.
Research from Childhood Brazil, a human-rights organization designed to protect children, shows sex crimes against youngsters increased by 66 percent during the 2010 World Cup in South Africa and by 28 percent during the 2006 games in Germany.
A network of Brazilian churches and nonprofit groups has joined forces to form Bola na Rede or "Back of the Net" in English, a nationwide campaign alerting tourists to the dangers facing the country's children.
"Over the last three years, we've been preparing churches in the 12 cities, encouraging them to mobilize their congregations so they actively do something in the days leading up to and during the World Cup," said Ronald Neptune, the national coordinator of Bola na Rede and a missionary with the United World Mission in Sao Paulo, referring to the 12 host cities.
"As Christians, we can't just clap our hands and praise the Lord. We have to work to make a difference to the lives of the young people at risk," he said. "We can be the eyes and ears on the streets and the motivating force that gets people out leafleting and speaking to tourists about how they can be vigilant to help protect our children."
On May 18, more than 97,000 Christians took to the streets in Brazil's host cities in a nationwide day of marches.
Outside the World Cup stadium in Itaquerao, Sao Paulo, where the opening match between Brazil and Croatia kicks off June 12, churchgoers gathered to protest peacefully with banners and posters. They then knelt in prayer for victims of sex abuse on the grass outside the stadium.
This coming week, evangelical churches will hold a series of daily prayer sessions on child sex abuse.
The Brazilian government is getting into the game, too. Last month, legislation was introduced increasing prison sentences and making those arrested for child sexual exploitation ineligible for bail. The government has also launched a series of advertising campaigns in English warning tourists that offenders will not escape prosecution.
"We need to keep our children off the streets," Neptune said. "We're supporting about 20 churches in each of the host cities who are planning to stay open with school vacation projects. They will provide Bible-related activities, games, sports and stories to occupy the children."
In Porto Alegre, in the southern state of Rio Grande do Sul, Joane Morais knows what it's like to be destitute. The 37-year-old mother of eight was sexually abused as a child and thrown out of her home when she fought off her attacker with a knife.
But Morais' life changed drastically last December, when women from a Baptist church knocked on her door, saying they were part of Bola na Rede and were working to protect vulnerable children and their families.
"Their involvement in my life made me realize the risks my children and others face," she said. "Now I've started going out to talk to other vulnerable mothers to teach them about God's love and the importance of protecting their children from the various forms of violence in our community, especially sexual exploitation."