A kindergarten teacher in China has been sentenced to death for poisoning dozens of children in an act of revenge against a colleague that left one toddler dead.

A court in the central Chinese province of Henan said Wang Yun put sodium nitrite into porridge being prepared for her colleague's students, sickening 25. The attack took place in March 2019 and left one boy severely ill for months before he died in January this year, according to news reports.

The Jiaozuo City Intermediate People's Court on Monday said Wang knew sodium nitrite was harmful but went ahead "with no regard for the consequences", leaving many innocent children in hospital. News reports last year said the children began vomiting and fainting after eating their breakfast.

Kindergarten students in China are aged between three and six. The death sentence was handed to Wang this week for the offence of using dangerous substances. Sodium nitrite is used for curing meats but can be toxic when ingested in high amounts.

Wang concealed her reasons for the poisoning after carrying out the crime and her "motives were despicable", the court said. Her "criminal methods and circumstances were exceedingly bad, with especially severe circumstances, and she should be severely punished in accordance with the law," the sentencing statement said.

The court added that Wang and the manager of the kindergarten must compensate the children's families. It was not the first time Wang used sodium nitrite to poison someone, authorities said. In 2017 she put it in her husband's mug, causing him minor injuries.

In March last year, 36 primary school students in southwest China's Sichuan province were hospitalised after eating "mouldy food".