Almost two dozen children were injured when a platform collapsed at a parkour gym for kids in San Diego, California, the Washington Post reports.

Vault Parkour usually specialises in parkour classes, but the ‘kid’s night out’ on Saturday is a bit of an all-out (the kids are supervised by Vault Parkour staff members, so it’s akin to a babysitting service).

About 150 kids were there last Saturday night thanks to a popular Groupon deal, but when a third of them rushed onto a wooden viewing platform – that’s where the pizza was being served – the stairwell leading to it collapsed.

It caused a pile of children and wood to fall in a heap, on top of more kids playing below.

Two adults (aged 72 and 46) and 21 children were rushed to San Diego-area hospitals with moderate of minor injuries; at least three had spinal injuries.

“It could have been much worse,” said San Diego Fire-Rescue Deputy Chief Steve Wright told media.

Building inspectors are now working to determine how the stairwell collapsed.

“We are sad to report that we will be closed for the time being,” the gym wrote on Facebook. “I️ am just as devastated for all the children affected as any other parent. Their safety has always been a priority. My heart aches for the families who call VaultPK home. We are working with authorities and will continue to do so to resolve this.”