Dina Rodrigues was found guilty of murder for orchestrating the June 2005 killing of 6-month-old Jordan-Leigh Norton - her then-boyfriend's child from a previous relationship.
Cheers erupted as the verdict was read in the packed Cape Town court room, where Norton family supporters wore pink in sympathy with the victim. Jordan's body, still dressed in pink, was found in a drain in Pretoria.
"We are glad for the decision," the baby's grandfather, Vernon Norton, told the South African Press Association, adding that the family hoped Rodrigues would be sentenced to life in prison at the next hearing in June.
The case has riveted South Africa - where an estimated 1,100 children are killed each year - in part because both the baby and Rodrigues were white and from privileged backgrounds. Most cases involving violence against children involve black and underprivileged defendants and victims.
High Court Judge Basheer Waglay also convicted four men of murder and robbery in the case.
The four had been hired by Rodrigues for a total of $1,500 to commit the crime, and posed as delivery men to gain access to the home of Jordan's grandparents, prosecutors said.
They stabbed the infant in the neck and tried to make the murder look like a botched robbery, prosecutors said.
Rodrigues' lawyer argued that it had been an attempted kidnapping gone wrong.
But the judge rejected the argument, noting testimony from the baby's father, Neil Wilson, that Rodrigues had phoned him to tell him she got rid of the baby.
Cases such as Jordan's have led to a recent campaigns drawing attention to the problem of violence against children.
Also Monday, five youths made a brief appearance in a Soweto court to face charges of raping, stabbing and stoning to death 14-year-old Thato Radebe, whose body was found on waste land.
"We have never had to deal with this level, intensity or numbers of crime against children," Childline head Joan van Niekerk said, noting her charity receives about 1 million phone calls from children reporting abuse each year. "Every year I start the year by saying it cannot get worse, and it does."
The newly formed Protect Our Children campaign donated 200 teddy bears Monday to a Pretoria police station to be given to young victims. Last week, the Solidarity trade union collected more than 1,100 bears - to mark the annual number of child murders in this country of 43 million - after the body of an 8-year-old girl was found in a sewer. AP-CS-05-07-07 1416EDT