The children of a shepherd family were playing in a mountainous village when they found the hand grenade and accidentally set it off, DC Buner Zariful Maani said.
The two brothers aged nine and 10 died, while their seven-year-old female cousin is in a critical condition, Maani said.
Another local government official, Aizaz Ahmad, confirmed the details.
8 dead as passenger van plunges into ravine in Buner
The village is close to the Swat Valley, where the army sent 30,000 troops in 2009 to battle militants of the proscribed Tehreek-e-Taliban Pakistan (TTP) led locally by Mullah Fazlullah. He had taken control of the valley and waged a campaign of violence, including beheadings and attacks on girls' schools.
Officials say that Fazlullah fled to Afghanistan during the offensive. In 2013 Fazlullah became chief of the TTP.
COMMENTS
Comments are moderated and generally will be posted if they are on-topic and not abusive.
For more information, please see our Comments FAQ