If you are travelling to or from South Africa with a minor (younger than 18 years old) you will need to produce an Unabridged Birth Certificate for the child in addition to their passport. This differs from some birth certificates you may have in containing the particulars of both parents.
This new government regulation was introduced by the Department of Home Affairs in 2015 to huge disappointment in the travel industry.
It is not a major obstacle. Full birth certificates are cheap and easy to obtain, but you must get them in advance to ensure that you comply with the travel restrictions. There are stories of families being refused entry to the country. Do not be one of them!
Whilst this requirement has subsequently been slated for amendment or removal, for the time being it still does apply to travellers departing from and arriving in South Africa. There are additional requirements if the child is travelling with only one parent, or without a biological parent, or is unaccompanied.
Please ensure you familiarise yourself with these requirements which are outlined in this document from the Department of Home Affairs.