San Diego's Vault PK is usually packed with bouncing and flipping children on Saturday evenings, when it hosts a "kids' night out" for budding athletes ages 5 to 14.
This past weekend was especially packed, parents said, as people cashed in a Groupon that got three kids into the parkour facility's open gym for just $30. The three-hour event is supervised by Vault PK staff members, so it doubles as a parents' night out too.
Some of the nearly 150 children present played on the America Ninja Warrior-styled obstacle course, but roughly a third had gathered on a 10-feet-by-30-feet wooden viewing platform, parent Cory Brizendine told San Diego ABC-affiliate KGTV. That's where the pizza was being served.
"Once the majority of kids got up there, the whole platform collapsed," he said.
The crumbling structure took a connected staircase with it, authorities and witnesses told reporters. Wood and little bodies tumbled to the ground — on top of children playing below — forming a heap of injured kids and gym equipment.
"It was business as usual until we heard a loud boom come from the gym, at which point our staff and some customers ran over to the gym to help any way we could," a spokesperson for Total Combat Paintball posted on Facebook. The business shares a building with the parkour facility and a cross-fit gym.
Zachary Smith, who was at Vault PK with his son for a birthday party, told the Los Angeles Times he was standing on the platform along with more than 30 others. Smith fell onto a young girl but neither were seriously injured, he said. Smith's son was also on the platform at the time but suffered only minor scrapes.
"It was a freak accident," Smith told the newspaper. He said it didn't appear the platform could hold so much weight.
No one answered the gym phone on Sunday afternoon. A recording said classes and birthday parties were "closed until further notice."
In all, 21 children and two adults, ages 72 and 46, were rushed to San Diego-area hospitals with moderate or minor injures, said San Diego Fire-Rescue Deputy Chief Steve Wright. At least three had spinal injuries.
Alerted to the collapse, parents who'd dropped off their children rushed back and found a street full of ambulances and firetrucks.
"It is a very serious night for parents," Wright said. "They had to wait to get in to see if their children were affected or not."
Vault PK, which also has a facility in Torrance, California, has been open since 2014. It was started by a former gymnast who saw a market for a gymnastics-centered facility that appeals to boys and girls.
Vault PK's classes, which start with preschoolers, include safety training. The challenges increase as students become more comfortable with their bodies moving over obstacles.
A promotional video shows people of all ages vaulting over obstacles, tumbling and flipping through the air as upbeat music plays.
"Train like your favorite super hero or ninja warrior!" says a description of the school's mini PK class. "Learn how to run super fast, leap tall buildings, climb walls, vault over obstacles like your favorite ninjas!"
Kids night out is a bit different. It invites children to "Ditch your parents and come run, jump, and play on our warped walls, trampolines, bars, and obstacle courses."
It's $17 for members, $22 for nonmembers, according to the facility's website. It's supervised by parkour coaches, but there's no instruction. Pizza is included.
Before Saturday, the gym had gotten great reviews.
"This place is fantastic for kids who are into lots of body movement, skater like moves, gymnastics and moving! It gives the kids all those opportunities but in a totally safe, controlled environment," one parent wrote in a testimonial on the facility's website. "Havent had a chance to do Kids night out yet, but i'm looking forward to it!"