The suspect, the school's former repair worker who was angry after he failed to pass his probation, hit the children's skulls one by one using a hammer, Hong Kong-based South China Morning Post quoted a former school maintenance worker surnamed Liang as saying.
Jia was a migrant worker from Heilongjiang province whose contract ended at the start of the year, and police suspect that the attack was motivated by the anger at having his employment terminated, the Post report said.
Several of the students were rushed to the intensive care unit of the nearby Xuanwu Hospital following the attack.
Three of the children have serious but non-life-threatening injuries, media reports quoted police as saying.
A teacher said "a worker at our school attacked 20 students in a first-floor stairwell. It is not like what everyone is saying, he did not use a knife," the report said.
A doctor posted on social media: "The mayor today came to the hospital to wait with us while the children were being medically treated. We have paused several operations to save all the children".
A man has been detained on suspicion of carrying out the attack, the Post report said.
Attacks on kindergarten schools and civilians in malls by disgruntled people have become common in China lately.
In January 2017, a farmer slashed and injured 12 children with a knife at a kindergarten in Youyi town, Pingxiang city. Four children were seriously injured while eight others received minor injuries.
The attacker was executed on Friday, the Supreme People's Court said Monday.
In October last year, a woman was detained on suspicion of stabbing 14 children at a kindergarten in Chongqing. In June, two children were stabbed to death outside a Shanghai kindergarten. Nine children died in a knife attack at a school in Shaanxi province in April.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)