The blast occurred in Kapisa province to the northeast of Kabul, the NATO-led International Security Assistance Force (ISAF) said in a statement.
Three other children were also wounded, the alliance said.
It did not give details about the type of the explosion, saying it was caused by insurgents who are fighting the Afghan government and foreign troops.
The South African and one of his Afghan colleagues, both employees of an Indian road construction firm, were killed by armed men in a southeastern area, said Sakhi Jan, an Afghan commander providing security for the firm.
An Indian and an Afghan worker from the company were wounded in the attack, which took place on a project site in Khost province, close to the border with Pakistan.
Violence has surged in recent years as the Taleban made a comeback, reaching its highest levels since the militants were ousted by US-backed Afghan forces in late 2001.
Record numbers of civilians and Afghan and foreign troops have been killed.
Washington has begun sending an extra 30,000 troops to join about 115,000 foreign troops, most of them American, already in Afghanistan in an attempt to reverse the growing momentum of the Taleban insurgency.
