The bomb exploded while the children were playing with it in Speenmark village in the tribal district.
"Six children aged between six to 12 years, all boys, were killed by a toy bomb and two others wounded critically," a local government official said.
Another local security official confirmed the incident and casualties.
The origin of the bomb was unclear.
South Waziristan is also one of the seven semi-autonomous tribal areas, where the army has for more than a decade been battling militants.
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