Climbing to the top of a hill in Golden Gate Park, 4-year-old Hugh Horner grabbed a piece of cardboard and rode it to the bottom of a concrete slide, coming to a stop in front of a pile of melted plastic and black ash.

The playground in front of him was sectioned off with yellow caution tape and a metal barricade, which he hoisted himself up on to peer through its bars.

The top of the green plastic slide was melted, almost a fourth of it on the ground in a hardened puddle. The smell of burned plastic still hung in the air of Koret Children’s Quarter, which many believe is the oldest public playground in the nation.

“Why would someone do that?” he asked his mom, Megan, who was standing next to him.

“I don’t know,” she said.

The playground’s platforms were burned black and hanging askew from the rest of the structure.

Police and firefighters responded to a fire just after midnight Sunday at the playground near the southeast corner of San Francisco’s urban oasis. The incident is still under investigation but is believed to have been the act of vandals. No arrests have been made.

The San Francisco Recreation and Park Department estimates the damage could total up to $1 million. Officials are waiting for a more thorough examination of the destruction before setting a timeline for repairs.

The playground was first opened in 1888 as the Sharon Quarters for Children. It underwent a major $3.8 million renovation in 2007 and reopened as the Koret Children’s Quarter.

The park’s swings, wave-theme climbing wall, concrete slides and a smaller play structure are still open. The fire caused major damage to the larger playground built for older children.

The playground is one of the most popular in the city and is packed on the evenings and weekends, said Drew Becher, CEO of the San Francisco Parks Alliance, the philanthropic group that helps promote and fund recreation and green spaces in the city.

“It’s like an amusement park without the lines,” he said.

Megan Horner brought Hugh and her three other sons to the park for the first time Wednesday. She said she and her family traveled to the city from San Luis Obispo and that friends had highly recommended she take her children to the Koret playground.

“It’s such a bummer,” she said. “I can’t believe this just happened.”

Instead of the larger playground, her sons played on the slides and smaller playground for much of their day visiting Golden Gate Park.

San Francisco resident Elizabeth Herrera said she comes to the playground with her sons — ages 5, 8 and 9 — at least a few times a week. Her boys love the rope climbing structure and were disappointed to see it taped off.

“This is the best park in the whole city,” Herrera said. “And that’s the most active part of the park for them.”

The community is already taking steps to raise money for the playground’s repair.

Children from Stepping Stones Preschool in the Inner Sunset were so distraught by the destruction that they came up with an idea for a lemonade stand to raise money to fix the playground. They set up Wednesday at Irving Street and Ninth Avenue, just south of Golden Gate Park, to sell lemonade and doughnuts.

Ruqoiyah Ghe-Huang, director of Stepping Stones, said the Inner Sunset community flooded the stand with support for the preschoolers.

“I thought we would make $10, but we made $275 in an hour,” Ghe-Huang said.

The children handed over the money to Mayor Ed Lee on Thursday at City Hall.

“This will help us repair our wonderful playground,” Lee told the preschoolers. “I know some people did some bad things to it, but you are all doing good things for us, so thank you. You have made my day.”

The Parks Alliance is also working with community members to raise money for the playground’s repair. Becher said the largest problem will be the bouncy surface the playground sits on, which was completely destroyed under much of the structure.

That will be the biggest cost, but Becher said he already has a few people who have committed to raising $25,000 for the park. The bulk of the money to cover the repairs will come from the Recreation and Park Department.

“Out of ashes usually come flowers and happiness,” Becher said. “So hopefully we’ll raise enough money to make it even better than before it happened.”

Chronicle staff writer Dominic Fracassa contributed to this report.

Alison Graham is a San Francisco Chronicle staff writer. Email: agraham@sfchronicle.com Twitter: @alisonkgraham