Of the 12 people hurt, six were adults and six children, it said, adding they were being treated in hospital.
A woman who was at the park with her daughter when the accident happened said she thought the ride might have been overcrowded.
"I was on a rope bridge and saw four or five rubber lifebuoys with people on them all crowded together and sliding down," the woman, who declined to give her name, said.
"The people were screaming, I'm not sure whether from fright or excitement, but within seconds they all flew off the slide."
She said it had been raining on the day and she thought the ride, which ran down a steep hill, might have been running faster than normal.
In a now deleted post on WeChat, the theme park said it had 32 slides of various sizes. A video posted on V.qq.com shows a young boy riding down a slide similar to the one on which the accident happened. The guard rails appear to be about 50cm (20 inches) high.
The park posted a notice on WeChat on Thursday saying it was closed for upgrading, but did not mention the accident.
Wednesday's accident is not the first at Children's Garden. Just two days after it opened in July 2017, three people were injured when a rope walkway they were on collapsed, Chengdu Economic Daily reported at the time.
News of the latest accident drew angry reactions on Weibo, with people questioning what had been done to improve safety at the park.
"I'm a local and have never gone there," a person wrote. "How connected is its boss? No one is regulating this?"
- South China Morning Post
