In recent years many boers have been under the "plasaanville" attacks. Also known as "farm attacks".
To my knowledge its a group of people, who are mostly black, attacking a, mostly white, farm. They're described as murders, rapes and other gruesome acts commited by the perpetrators.
Read more about them here:
https://en.metapedia.org/wiki/South_African_farm_attacksFor the lore of Coco, its still very unknown if her parents were actually killed by these, but to me its seems like the only plausible reason.
Why the niggers let Coco go, I do not know, but whats for sure is that her parents died.
Make your own conclusions, we only have 20 minutes of lore anyways, its not like anyones gonna blame you, but almost everyone in the zellig fandom theorizes that the plaasaanville are the reason behind Coco becoming a biological parentless child.