It is thought that the two girls ingested rat poison which had been put in the yogurt drinks by the owner of a rival nursery school in Hebei province.
The girls were said to have suffered convulsions and started foaming at the mouth after drinking the yogurt late last month.
The Xinhua news agency says that the owner of a nursery school has confessed to poisoning the yogurts because the two schools were competing to get children on their roll in the village of Lianghe, Pingshan county.
A man who allegedly helped the woman plant the yogurts in the rival school has also reportedly been detained by police.
The news agency says the grandmother of the little girls involved found the drinks and several school notebooks on the roadside close to their school. She took it home and gave it to the girls.
The BBC says China has seen a string of cases involving tainted food and medicine in recent years. Five years ago, tens of thousands of babies were left ill after drinking infant milk powder which had been laced with the industrial chemical melamine. Six children died after consuming the milk.