Initially, I heard that a klonkie was a black playmate for a white kid in South Africa. Each kid would often have a black playmate growing up who was a child of one of the servants, and they would be somewhat inseparable, but as they grew up, they would grow apart from one another.

The Dictionary of South African English, though, says:

klonkie, noun
/ˈklɔŋki/

Origin:
AfrikaansShow more

derogatory, offensive
Of black or ‘coloured’ people: a patronizing name for a youth; an insulting name for a man; klong.
1953 A. Paton Phalarope (1963) 25The klonkies there, the small black boys, having learned it from the soldiers who camped in Venterspan during the war of 1939, saluted him.
Show more
1989 J. Hobbs Thoughts in Makeshift Mortuary 277‘The klonkie up the ladder?’ He jerked his head at Jake. ‘He’s just a painter who was recommended to us.’

Klong is also interesting:

klong, noun
/klɔŋ/

Origin:
See klonkie.

offensive
klonkie.
1913 C. Pettman Africanderisms 268Klong,..The word is in common use in various parts of South Africa, and is applied to coloured males without reference to age, much as the word ‘boy’ is among the English colonists; indeed so far has the original sense disappeared that the expression ‘ou’ klong’ (lit. ‘old small youngster’) is by no means uncommon.
1963 A.M. Louw 20 Days 71‘He is a big klong now. I can’t work for him any more. He must work for me. What does a mother bring children up for?’ she said.

Very interesting material - check out more here:

Home - Dictionary of South African English

Dictionary of South African English - Over 4000 uniquely South African words across three centuries, browsable by category.

^dsae.co.za