ZHENGZHOU, April 25 (Xinhua) -- More than 50 children were poisoned by a kindergarten breakfast in central China on Wednesday morning.

The children in Xinxin Kindergarten, a private one in Zhengzhou City of Henan Province, complained of stomach ache and some of them began vomiting about half an hour after breakfast, for which they had soy-bean milk and fried eggs and cucumber.

The children were immediately taken to hospital. Thirty have been discharged and the others remain under observation.

According to initial test results, the cause of the poisoning was that the soy-bean milk was not boiled properly, said doctors with Zhengzhou First People's Hospital.

The person in charge of the private kindergarten and three cooks who prepared the breakfast have been detained by police.

An investigation into the accident is continuing.