The incident took place at Multan district, some 350 kilometres from Lahore.
"Four young boys were playing near a canal when one of them picked up a grenade lying on the ground and opened its pin. It exploded and caused injuries to all of them," senior police officer Khalid Rauf told reporters.
The injured were shifted to Nishter Hospital where the condition of a 14-year-old boy is stated to be critical. The boy had picked up the grenade, Rauf said, adding a couple of sheep also died.
The police have launched a probe into the incident, Rauf said.
Yesterday, a father and his two children died when a toy bomb exploded in Pakistan's restive northwestern tribal region.
