Citizens of the USA, Canada, United Kingdom and Australia do not require a visa to enter South Africa as a tourist for stays of less than 90 days.

To enter South Africa, a passport valid for six months beyond the end of your intended stay, with at least two blank pages, is required.

http://www.home-affairs.gov.za/index.php/immigration-services/types-of-visas

IF YOU ARE TRAVELLING WITH CHILDREN (<18 yrs) PLEASE NOTE, you require specific types of documentation to enter South Africa, without which you will be denied entry. Different documents are required for situations where; both parents are traveling with a child/ren, one parent, divorced parent/s etc please consult the department of home affairs for your specific requirements

Mandatory requirements for parents travelling with children below the age of 18:
- Certified unabridged birth certificate for all minors.
- A letter of consent from the other parent should the minor be travelling with only one parent.

http://www.dha.gov.za/index.php/civic-services/traveling-with-children