Organisers of the religious pilgrimage gave a higher toll of 40, with 31 injured.
The accident occurred on Sunday when the speeding bus overloaded with young adherents of the Tijjaniyya Sufi order lost control and crashed into a truck in Lere district, Kabiru Nadabo, head of the local office of Nigeria's road safety agency, FRSC, said.
"The bus was overloaded with 63 children and the driver was speeding recklessly when he lost control and rammed into an articulated truck," Nadabo said.
"Fifteen of them died on the spot while 48 injured were taken to various hospitals, among which 10 died the following day, raising the death toll to 25," he said. The children were from Kwandare village and heading to the nearby town of Saminaka for the annual Maulud festivities marking the birth of the Prophet, said Nadabo.
He said the death toll could have changed since the injured were taken to hospitals in various locations and he did not get further updates.
Dikko Dahiru, one of the organisers of the trip, said 40 children were killed in the accident, while 31 were injured. "The bus was carrying 71 passengers and 36 died instantly while four more died in hospital the next day," said Dahiru, whose nephew was among the dead.
"Thirty-one were taken to hospitals with severe injuries, 11 of them in critical condition," he said.
Road accidents are common on Nigeria's poorly maintained roads due largely to speeding and disregard for traffic rules.