Taliban insurgents claimed responsibility for the attack, saying it was launched against a "gambling match where local police were present".
Gambling on animals forced to fight -- from quail and roosters to camels, rams and bulls -- is popular in
Afghanistan but was banned by the Taliban during their rule from 1996-2001.
The motorcycle-bomb exploded at a bird fighting pit in the Greshk district of restive Helmand province where a crowd of more than 100 people had gathered to watch the regular match, said local police chief Shadi Khan.
"The explosion has killed a 13-year-old boy and wounded eight other people, all of them innocent civilians," he said.
In 2008 a suicide blast hit a crowd of men watching dog fighting in neighboring Kandahar province, killing up to 80 people and wounding dozens more.