Ten children were rushed to Arkansas Children’s Hospital in Little Rock after accidentally drinking windshield wiper fluid at a day care center, authorities said Friday. The incident happened after a staffer served it from a container mistaken for Kool-Aid and placed in a refrigerator.

Laura James, a pediatric pharmacologist and toxicologist at the hospital, said the children, aged 2 to 7, drank about an ounce of the blue fluid late Thursday afternoon before realizing  it tasted wrong. Fortunately, the children suffered no visible side effects. Only one of them had to remain hospitalized Friday morning, after tests showed “measurable levels’” of methanol in his blood. Methanol is a highly toxic alcohol that can induce comas and cause blindness.

 After the incident the day care operator, Carolyn Bynum, surrendered her state license. The license had allowed her to take care for 10 children in her home in Scott, near Little Rock.

“She was so upset about what had occurred and she was definitely worried some of the children had been injured. It was just a mistake, she says. She says it was just a horrible mistake,” said Julie Munsell, a spokeswoman for the state Department of Human Services.

Munsell also added that Bynum has no prior history of any violations with the daycare run out of her home.

The Human Services Department on Friday continued to investigate the circumstances surrounding the incident. Officials say there is no indication so far that the children were poisoned intentionally.