Police said the child died after its mother was shot dead by up to seven gunmen who attacked two homesteads on Thursday night, killing another six women and three men. Most were shot at close range.
Police said the attack on the rural settlement, near the KwaZulu-Natal town of Donnybrook was politically-motivated, as all those killed were ANC supporters.
The massacre took place after Mr Mandela told meetings in the KwaZulu-Natal province, which is ruled by his ANC's main rival, the Zulu-based Inkatha Freedom Party, that blacks were still slaughtering one another there.
The provincial police commissioner, Chris Serfontein, said police were offering a reward of 250,000 rand (pounds 42,000).
Mr Mandela said at one of his meetings on Thursday that there was peace between "Indians, Coloureds [people of mixed race] and whites but it is Africans who are slaughtering each other".
"Many people in the rest of our country still regard us as being backward. What else can they say when you are behaving like animals, when you slaughter each other simply because you want to remain in power?"
Makubetse Sekhonyane, from the watchdog Human Rights Committee, said that the attack could be linked to the launch of the ANC's campaign for the 29 May local elections in the province.
"The leaders of the two political parties have just had a pre-imbizo [gathering of the Zulu nation] meeting to discuss peace, but this attack shows just how far away peace really is."