"Today is the end of the journey," she said.
Nkosi's funeral on Saturday, eight days after he died of the disease, attracted more than 1,000 mourners to Johannesburg's central Methodist church, where his body lay in a white coffin.
The service included hymns in English, Zulu and Xhosa inside the church, while outside hundreds of Aids activists waved placards demanding better treatment for the millions of Africans suffering from the disease.
Nkosi's maternal grandmother Ruth Khumalo thanked his white foster mother Gail Johnson for adopting the two-year-old orphan when his mother died of Aids.
"Today is the end of the journey," she said.
Addressing the mourners, Gail Johnson said: "Nkosi taught me unconditional love and acceptance. I ask South Africans to do the same."
The congregation included many Aids activists, entertainers and politicians.
Among them was Zambia's former president Kenneth Kaunda, but neither South African President Mbeki nor any of his senior ministers was present.
Government under fire
Government policy on Aids came under strong attack from the presiding Bishop of the Methodist Church in southern Africa, Mvume Dandala, during the funeral service.
"I don't care what position you take in this debate on Aids. I don't care what position you take on anti-retroviral drugs. Just show this country, show these children some compassion," he demanded.
The service, filled with songs and praise for Nkosi, went on for nearly three hours.
At a memorial service on Wednesday pastor Jonas Khauore, who works with young children in Soweto, said: "I believe that God has used Nkosi to make people aware of the Aids epidemic... Through his talks and boldness he could influence the decision makers."
Perhaps more than anyone else, Nkosi became the public face of the fight against Aids which is devastating much of Africa.
Former South African President Nelson Mandela praised Nkosi as "exemplary in showing how one should handle a disaster of this nature".
The Star newspaper in Johannesburg wrote: "There cannot be a South African who has not been touched by little Nkosi.
"But all around us are so many Nkosis, and because this is the natural way of things, they are forgotten."
'Dying country'
An estimated 200 children are born HIV-positive in South Africa every day, and there are believed to be 300,000 Aids orphans. There are fears that number could reach two million by the end of the decade.
BBC Johannesburg correspondent Allan Little said that Nkosi had succeeded in shaking South Africa, where one in nine people are HIV positive, out of a state of deep denial.
"You can't get Aids by hugging, kissing, holding hands. We are normal human beings, we can walk, we can talk," Nkosi told delegates in Durban.