It sure looks nice and, with any luck and with at least four guards on duty around the clock, it might even stay that way.

That’s what the customers and their parents were saying about the brand new playground in front of San Francisco City Hall, which underwent its first full day of swinging and sliding on Thursday.

It’s called the Helen Diller Civic Center Playgrounds because it’s actually two playgrounds, one located in the northeast corner of the plaza and one located in the southeast corner. Together they cost $10 million, or about three times what it cost to build City Hall in 1913.

Each playground has a long, fast slide and a large climbing structure that could pass for a flying saucer crossed with a trampoline. There are a few swings to keep the traditionalists happy. There are also some arty, illuminated sculptures that kids are allowed to admire but not climb or clamber on, which runs somewhat counter to the playground ethic.

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Jackson Bingham, 5, dropped by on opening day with his adult big sister, Ashley. He said it was his first time on a tire swing. Diller, the late San Francisco philanthropist and health care advocate whose family foundation donated the $10 million to fund the new playgrounds, believed in things like children, civic duty and, probably, tire swings.

“I like it,” Jackson said. “I’m going to come back and do it some more.”

Ashley said that would be fine with her as long as the city maintains the playground.

“That will take guards and police,” she said. “I hope they’ll be out here.”

One of the guards, Louie Hammonds, said his job is to keep the playground free of that fabled Civic Center trio of trash, needles and poop. Also unwelcome is that human subspecies known as an “unaccompanied adult.”

At movie theaters, kids are to be accompanied by adults. At playgrounds, adults are to be accompanied by kids.

Dahlia Paul, 4, said she liked the flying saucer things, but that there was something wrong with the grass, even though it was green.

“It’s pretend grass,” she said, running her fingers through the plastic. “Real grass is softer, if you fall down.”

However, pretend grass comes with a safety pad underneath, needs no mowing and “conserves water,” said Connie Chan, spokeswoman for the city Recreation and Park Department.

On opening day, kids were learning a lesson about static electricity, because the fake grass was acting like a carpet and kids were receiving a steady dose of harmless shocks when they touched the metal poles holding up the flying saucer-trampoline thing.

“We’re telling everyone it’s shockingly exciting,” said Hammonds.

Sadie Franklin, 8, a student at Marin Preparatory School on a field trip, said the flying saucer is great.

“You can climb on it and you can go underneath it and if somebody starts chasing you, then you can go down the slide,” she said, and then she did exactly that.

At the grand opening Wednesday night, grown-ups stood on a stage and made speeches about the new swings, while bundled-up kids actually rode on them. Mayor Mark Farrell called the new playground a “great equalizer.” Downtown neighborhoods have lots more kids these days, searching for things to slide, swing and climb on.

Ari Dworman, 2, said the new swing he was riding on was “fun” and “not scary.”

His mom, Corinna Lu, said the swing isn’t scary but the people she sees injecting themselves while seated on Civic Center benches certainly are.

“People shoot up right here all the time,” she said. “If they have enough guards, it could be all right. It will take two guards at each playground.”

The two playgrounds are part of $30 million in downtown park improvements by the Recreation and Park Department that include a $9 million renovation of Boeddeker Park, a $4 million batting cage at Draves Park and a $3 million remodel of Macaulay Playground.

The Diller playgrounds were designed by the Andrea Cochran Landscape Architecture firm of San Francisco, which adorned them with a forest of 52 lighted poles that glow invitingly at night, well past most 5-year-olds’ bedtimes.

On Thursday, two police officers, two park rangers and four private security guards were standing watch at midday, making the new playgrounds probably the safest place to be in San Francisco. The playgrounds will be monitored during the night to make sure that adults — accompanied or unaccompanied — stay out.