If a child is travelling alone or with only one parent, in many cases it will require its parents' permission to travel, or the permission of the parent who is not travelling. The authorities may refuse entry or exit without this document. In some cases, the airline and/or the airport will also receive a monetary fine.
Immigration regulations for children
Lots of countries, such as South Africa, have special immigration regulations for children. Make sure that you have all the required documents ready before your trip.