San Francisco’s newest kiddie playground comes with a slide, a jungle gym, a dozen mosaics of butterflies — and three uniformed guards.

You have to be a kid to get in. If you’re a grownup, you must be accompanied by a kid. That’s what the signs say at plenty of San Francisco playgrounds. But at the $2.2 million Turk-Hyde Mini Park, in the heart of the Tenderloin, the signs mean business.

The park — a quarter-acre retreat from what takes place across the street and down the block — has been packing in scores of toddlers, parents and nannies ever since it reopened last week after about a year of what park planners call “reimagining.”

It’s clean, it’s pleasant, it’s safe — and it’s going to stay that way, vowed head guard Dewayne Kemp, his arms folded across his chest, as he gazed through the black metal fence separating the children from the tent-and-syringe scene across the street.

“We want everyone to feel safe here,” Kemp said. “We pressure-wash the park every morning. No needles. No litter. No unaccompanied adults.”

In the middle of the afternoon one day last week, after schools let out, dozens of kids streamed past Kemp, through the gate in the tall black fence, heading straight for the slide. Kids often navigate Tenderloin sidewalks in long lines of chaperoned school groups, holding ropes to keep them together, staying quiet. Once they arrive at places like the Turk-Hyde Mini Park, they turn back into kids.

“It’s a good slide,” said Carlitos Vasquez, 6, after his third descent. “Fast. I like going fast.”

His mother, Nina Vasquez, said she liked the new park because it’s spotless, it’s closer to her residence than the Civic Center playgrounds (which also have uniformed guards) and it “hasn’t been messed up yet.”

“This park,” she said, “is a little break from everything that goes on all around here. You don’t have to worry about stepping in things you don’t want to be stepping in.”

Across the street, two sidewalk tents were going up. Half a block south on Hyde Street, three men sitting by the curb were passing around a small pipe.

Another uniformed guard, Don Naly, said the park has been a great success so far, and the guards intend to keep it that way. Without the guards, he predicted, the park would quickly be overrun by big people with bad habits.

“Within a week, it would be chaos,” he said. “Gangs would take over. The children would all be gone.”

Being a Turk-Hyde Mini Park guard calls for diplomacy, Naly said. They respectfully ask smokers and imbibers to move a few steps farther down the sidewalk, and usually they do. If people linger at the Muni stop without getting on the 19-Polk bus, he said, the guards respectfully ask them to do their lingering someplace else.

The small park had been closed while the city installed a state-of-the art German play structure, a long mosaic bench, four tables, seven trees and two swinging benches for the parents and nannies. Missing are bathrooms. For that, the guards say, there is the Pit Stop public toilet trailer half a block south.

The rules are posted on three big green signs attached to the gate. No camping, smoking, bird feeding, littering. No alcohol or pets. No drugs, skateboards, scooters or bikes. Nothing with wheels — baby carriages excepted.

Per park code sections 3.21, 3.12, 3.02, 4.01 and 4.04.

“The Tenderloin is one of the densest areas of our city yet has the least amount of open space,” said Simon Bertrang, executive director of the Tenderloin Community Benefit District, which is paying the guards through the Urban Alchemy program that also staffs the Civic Center playground and downtown BART elevators.

Neighbors say the new park is a step forward, one of many breaks the Tenderloin needs and often doesn’t get.

“It’s a great park, so far,” said Mohammen Musla, a cashier at the Hyde-Turk convenience store across the street. “We’re selling a lot of ice cream to kids, and getting new customers. I like it.”

Barbara Meyers, a minister with the Sidewalk Talks group, set up her folding chair just outside the park fence and chatted up passersby on the pavement. Even a servant of God can’t get into San Francisco’s newest mini park, she said, if she’s not a kid or not accompanied by one.

Steve Rubenstein is a San Francisco Chronicle staff writer. Email: srubenstein@sfchronicle.com