The children are mostly Aids orphans living in the Malerato Centre for Hope in Mamelodi township outside Pretoria.
They were said to have complained of severe stomach aches on Thursday,
The orphanage’s principal, Johanna Mashapa, told local media the children had been given a powder with their lunch.
“We were so worried. They were vomiting and crying. They had runny stomachs and were so sick,” she told South Africa’s Sunday Times newspaper.
Police say a woman has been charged with assault with intent to cause grievous bodily harm and is due to appear in court on Monday.
The Malerato Centre houses more than forty abandoned children and orphans whose parents died from Aids.
Two of the children were reported to have been found in a critical condition; one of them was airlifted to a Johannesburg hospital.
Government inspectors were sent into the orphanage to investigate.