The children, aged three and four, and adult staff members of the Growing Tree Learning Center in Jersey City, US, were taken to hospital in five ambulances after they complained of light-headedness and burning in their stomachs.
The children and staff were having a morning snack of water and fruit when an adult noticed a faint bleach smell and reported it immediately.
Keith Kearney, executive director of United Cerebral Palsy of Hudson County, which runs the day care centre that is open to all community members, told local news: "We had an incident where one of the staff in the kitchen used the cleaning solution bottle to pour water for some of the children.
"It was a re-purposed plastic milk jug with a bleach water solution we use to wipe down surfaces.
"The label we had on the bottle wasn't large enough to guard against it." Thankfully, all 30 were in stable condition and many were discharged.
A paramedic said: "There was a poisoning and many children were injured."
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Doctors gave the victims liquids like water and milk to dilute the poison rather than inducing vomiting 'because it would burn on the way back up again'.
Jersey City Medical Center's Michael Bessette said: "It was in a low concentration so it didn't cause any damage to the children.
"They're obviously a little excited, they get a big field trip today."