Seventeen had to be transported to local hospitals.
Nine of them were taken to Huntington Hospital in Pasadena. All the children were treated and released, except for one that will be kept overnight.
Eight of them were transported to Methodist Hospital in Arcadia. Five of them have been released, while the other three remain under observation.
The emergency call to Arcadia County Park on South Santa Anita Avenue went out at about 11:30 a.m.
Dozens of children were at an open swim session when a pump at the pool malfunctioned, leading to the pool becoming over chlorinated, authorities said.
The kids and some life guards began complaining of shortness of breath, problems with breathing, burning eyes and throats.
"There's only one (child) that I know of so far who may have had underlying respiratory issues anyway," said Dr. Roy Antelyes of Huntington Hospital. "He's going to be admitted to the hospital."
"The children who were swimming in the pool, and some that were out of the pool, noticed the color of the chemical in the water and got out of the pool," Antelyes added.
Authorities said it was good that the children were attending the pool with their parents.
"Largely, the parents were already with their children," said Beth Stogner, a spokeswoman for the Arcadia Fire Department. "I think that helped contribute to the calm atmosphere that we encountered when we arrived."
Some parents who use the pool regularly said the pump is cause for concern.
"He was asking me if I'm going to bring my kids back and right now it's a definite no," said parent Sandra Pinedo. "It's scary."
Hazmat crews were called in to decontaminate the pool.