If anyone can upstage South Africa's living legend, Nelson Mandela, it's the Children of Agape. Against the pain of losing their parents to Aids, these 15 children sing and dance with a raw, pulsating energy. This week they will join the party in London to celebrate Mandela's 90th birthday. Their harmonies lift the spirits; their solo voices are searing. Throw in the stars of the West End show The Lion King, who met the South African choir in a “sing-off” last weekend, and the Lyceum Theatre's walls threatened to burst.

The Children of Agape choir came to prominence this year in We Are Together, a documentary shot by Paul Taylor, a 25-year-old British student, who did voluntary work at the orphanage in the Valley of a Thousand Hills, near Durban, in his gap year. As reported by The Times, the film was painstakingly made over three years, against all the financial odds. Taylor's camera became a friend to the children and his film has won awards and enthusiastic reviews.

Now high-profile international fundraisers are converging on this choir. Bono, Alicia Keys, Paul Simon... everyone wants to be part of their story.

What strikes one is their poise and grace. They sing with honesty: no fuss, no nerves, no showing off - they just stand and deliver. Such startling self-possession is shaped by their tragic circumstances. They are among an estimated 1.2 million Aids orphans in South Africa, victims of a pandemic that has left almost six million living with Aids/HIV.

Pamela Mqadi is Agape's general manager, a maternal figure ready to comfort, cajole and control this lively group. “The children have been through many things and they have always adjusted,” she explains. “They've lost their parents, their relatives to Aids - they are used to whatever can happen to them.”

Slindile Moya was orphaned eight years ago, at the age of 9. She and her three siblings - two sisters and a brother - form the heart of Agape's choir and for them singing is a poignant link to their parents. “Sometimes when I sing, I feel sad,” Slindile says. “I remember my parents because they are the people who taught me to sing.” Slindile and her older sister Nonkululeko are beautiful, graceful young women, generous with their personal memories. “Our parents were Christians so we used to sing all the time,” explains Nonku, at 28 the eldest in the choir.

Other children learnt to sing at Agape, attending daily practice under the strict but encouraging guidance of the musical director, Thembelani Luwaca. “One thing that's amazing about these kids is they put music first,” Luwaca says. He has nurtured their talent from the cramped orphanage to the international stage. “Even if someone is frustrated, they put on a good-looking face for the choir. They enjoy music.”

Exuberance underpinned by tragedy has proved a potent fundraising force. Sales of the choir's soundtrack CD, We Are Together, and proceeds from the film, paid for the orphanage to be rebuilt after it was destroyed by fire. The new, bigger Agape is home to 45 children aged from 1 to 18. Proceeds have also paid for 39 of them to attend a better school.

These children have lived a whirlwind few years of triumphs, disappointments and rapid change. Mbali Mqadi, 11, isn't sure if she's been to England before, though she remembers visiting New York. (The choir will make a fifth fundraising visit to America this year.) Mbali is bashful, whispering answers - until I ask her about singing. Suddenly a great white toothed-smile spreads across her face. “I love to sing because I have a nice voice,” she says, beaming proudly. “And when I grow up, I want to sing.” What songs? “Zulu songs.”

The starriest ambitions remain defiantly rooted in home. Seven-year-old Mtho Moya is the youngest member of the choir. A boisterous four-year-old when Paul Taylor shot his documentary, Mtho has grown into an intriguing little boy: wise one minute, with a furrowed brow as he sings; cavorting in headstands and wriggling on his stomach the next.

He says he likes all the attention but not for the reasons one might expect. “I like to be famous so I can help people who are poor,” he says. And does he still enjoy singing? “Yes, I like singing because it has built Agape new houses after the fire.”

At school these children are “the orphans”, different and open to teasing. When Nonku Moya went back to school this year, at the age of 28, her classmates singled her out, puzzled why someone so old should be in their lessons. “They didn't know I lost my parents and that's why I was forced to leave the school. Now they understand my situation and they know that somebody is helping me go back to school.”

While life has made these children resilient, singing in the Agape choir has taught them self-esteem and lifted them out of a vast, tragic pool of statistics. “They don't look down on themselves any more,” Luwaca reflects. “Now they regard themselves as humans, more than the fact that they are orphans.”

“Being in the choir has given us confidence,” Slindile Moya says with a shy smile. “I feel proud of being talented and proud of what we have.”

We Are Together is released on DVD on Aug 4. The soundtrack CD is on EMI. All proceeds go to the Rise Foundation for the Children of Agape (wearetogether.org). Nelson Mandela's 90th birthday concert is on Friday in Hyde Park (46664.com)

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