They should be playing, learning and enjoying their childhood. But instead these youngsters, some as young as five, are being married off in secret weddings. It is estimated that every year this happens to ten to 12 million girls in the developing world.
In India, the girls will typically be attached to boys four or five years older, an investigation in the June issue of National Geographic magazine has found. But in Yemen, Afghanistan, Ethiopia and other countries with even higher rates of marriage at an early age, the husbands may be young men, middle-aged widowers or even abductors who rape first and claim their victims as wives afterwards.
'Whenever I saw him, I hid. I hated to see him': Tahani (in pink) was just six years old when she she married Majed, 25 (standing next to her). The young wife posed for this portrait with former classmate Ghada, also a child bride, outside their mountain home in Hajjah, YemenSome of these marriages are business transactions or to resolve a family feud.
Forced early marriage thrives in many regions, often in defiance of national laws. Whole communities often prescribe to the notion that it is as an appropriate way for a young woman to grow up when the alternative is the risk she loses her virginity to someone before she marries.
Wedding ceremonies are often held in the middle of the night, with the whole village keeping the secret for fear there might be a police raid.
In a project for National Geographic magazine, journalist Cynthia Gorney and photographer Stephanie Sinclair travelled to Yemen and Rajasthan in India to investigate the extent of this shocking practice.
In India girls may not legally marry before the age of 18 - but ceremonies involving girls in their teens may be overlooked. The younger daughters, some aged five, tend to be added on discreetly, their names kept off the invitations.
In one case in Rajasthan where her teenage sisters were also being married, a five-year-old bride named Rajani fell asleep before her wedding ceremony began.
Well past bedtime: Child marriage might be illegal in India, but this doesn't stop ceremonies taking place in the small hours of the morning - it becomes a secret the whole village keeps, explained one farmer. Here, five-year-old Rajani is roused from her sleep long after midnight and carried to her wedding by her uncle
So young: Barely looking at each other, Rajani and her boy groom are married in front of the sacred fire. According to tradition, the young bride is expected to live at home until puberty, before she is transferred to her husband
Three years ago, the case of Nujood Ali came to worldwide attention. The ten-year-old Yemeni girl managed to escape her home and made her way to a courthouse to request a divorce from the man in his 30s her father had forced her to marry and who beat her.
She became the poster girl for children in her position around the world and a recent book, translated into 30 languages - I Am Nujood, Age 10 and Divorced. She is now back with her family and has returned to school.
Not all girls have such a lucky escape. Few who are married off as children have any chance of an education but there are far worse consequences.
Many are raped and have a low life expectancy due to the number of children they carry at such a young age.
Girls suffer physical abuse and are too frightened to escape because they are threatened with death.
In another case in Yemen, it was discovered that a ten-year-old girl Ayesha had been married off to a 50-year-old man.
The journalists were told by her sister Fatima that 'little Ayesha screamed when she saw the man she was to marry'.
Wailing in protest: Surita, 16, cries as she leaves her family home, shielded by a traditional wedding umbrella and carried in a cart to her new husband's village. Early marriage is the norm in her small village in Nepal
Someone alerted the police, but Ayesha's father ordered her to put on high heels to look taller and a veil to hide her face.
He warned that if he was sent to jail, he would kill Ayesha when he got out. The police left without troubling anyone and Ayesha now lives in a village two hours away with her husband.
'She has a mobile phone,' Fatima said. 'Every day, she calls me and cries.'
The medical consequences are also extremely serious and in some cases fatal.
One doctor based in the Yemeni capital Sanaa listed some of the medical consequences of forcing girls into sex and childbirth before they are physically mature - ripped vaginal walls and internal ruptures called fistulas which can lead to life-long incontinence.
Girls are often too young to understand the concept of reproduction. The doctor said: 'The nurses start by asking, "Do you know what's happening?" "Do you understand that this is a baby that has been growing inside of you?''
Few are equipped with the information of how to care for themselves or their babies after childbirth leading to high infant mortality rates.
The people who work full-time trying to prevent these illegal marriages, and to improve women's lives, know that it is a far from simple plan of rescuing girls.
Molly Melching, the founder of a Senegal-based organisation Tostan, told National Geographic: 'If we separate a girl and isolate her from her community, what will her life be like?
'You don't want to encourage girls to run away. The way you change social norms is not by fighting them or humiliating people and saying they're backward. We've seen that an entire community can choose very quickly to change. It's inspiring.'