(CBSChicago) — Two children were shot on a playground outside a South Side elementary school, innocent bystanders of a drive-by shooting.

The children, two girls ages 7 and 13, ran into Warren Elementary School after shots rang out around 1:45 p.m., according to community activist Andrew Holmes. One of the children was shot in the hand, Holmes said. The other was shot in the leg, according to police.

Their conditions were said to be non-life threatening. The 13-year-old was being prepared for surgery, according to the girl’s sister, Daytonna Hart, who was at the hospital.

“She’s in a lot of pain,” she said.

Police Supt. Eddie Johnson said the shooters were firing at two former students who had tried to crash a cookout at the school. They were denied entry and were hanging outside school grounds when a car drove up and somebody started shooting.

“We all hit the floor, it was a drive by,” according to one witness.

“This makes me sick,” Johnson said. “That’s not what we are about.”

The girls were playing at Warren Elementary School, 9239 S. Jeffrey, in the Calumet Heights neighborhood, which was celebrating with a cookout at the time of the shooting.

“Numerous casings” were found at the scene, according to police scanner traffic.

According to premliminary information, police have “persons of interest” in custody.

Chicago Mayor Rahm Emanuel went to Comer Children’s hospital to check on the victims and visit with the family.

“This is a parent’s worst nightmare,” a visibly angry Emanuel told reporters. “Those kids were out on the playground enjoying their childhood.”

“There is no person in Chicago who is not disgusted by this incident. They deserve their childhood uninterrupted.”

“I would hope for once, when people say enough is enough … that the justice system understands that enough is enough.”