The driver and a female teacher also died in the inferno, which erupted inside a tunnel as the children headed to an international school in Shandong province on the morning of May 9.
Chinese officials said the children were aged between three and six, with five from South Korea and six from China. But the South Korean embassy has said that 10 were South Korean, including five with dual citizenship.
The government of Weihai city, where the tragedy occurred, said the investigation concluded that the blaze was a case of arson committed by the driver.
"The driver was disgruntled because his overtime and night shift allowance were stopped, causing his income to plummet," the city government said on its Weibo microblogging account.
The driver had brought gasoline onto the bus, the government said. The fire was located near the driver's seat. A lighter cap was found nearby along with gasoline residue.
Authorities ruled out a short circuit or a traffic accident as other potential causes.
But South Korean family of some of the victims took issue with the official findings that the driver was to blame.
"The unconvincing explanation by the Chinese government gives an impression that they were trying to frame it as a fault of the driver," it quoted Kim as saying.
The driver was found in the middle of the passenger seats, suggesting he might have been trying to rescue children trapped in the back of the vehicle, Yonhap said.
Another victim's father, Lee Jung-Kyu, said the investigators' characterisation of the driver as "mentally deranged" did not ring true.
"I know he was such a nice, delightful person who always greeted the children and their parents...he's not a person who could do such things to children he has known for more than two years," said Lee, adding the families would demand the case be reinvestigated.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)