The incident, which comes after a series of similar attacks by anti-poll protestors in recent days, occurred when an eight-year-old boy picked up the bomb apparently thinking it was a toy, another police official said.
"Eight-year-old Samir Khadgi grabbed the bomb assuming it to be a toy on a road in Bhotebahal," Hemant Pal, a Kathmandu-based police official, told AFP.
"He has injuries on his hand and face," Pal said.
Two others suffered minor injuries, local police official Yagyadinod Pokharel told AFP.
Demonstrators from a hardline faction of the Maoist party, which won elections in 2008, have escalated efforts to disrupt the vote in recent days, torching vehicles and hurling explosives at traffic.
The protestors say elections cannot be held under an interim administration set up after the collapse of Nepal's first constituent assembly, and want polls to be postponed until a cross-party government is put in place.