The second child, a five-year-old boy, is currently fighting for his life in hospital.
Police were called to Family Golf on Sidcup Bypass at around 5.45pm on Sunday (November 9).
When officers arrived paramedics were treating both children, before they were taken to hospital.
The girl has since been discharged, while the boy is in a life-threatening condition.
Police enquiries are ongoing, with Family Golf remaining closed and a heavy police presence sighted at the business the following day.
An eyewitness told News Shopper that the boy had been attending a birthday party when the wall collapsed.
Family Golf has been contacted for comment.
A London Ambulance Service spokesperson said: “We were called at 5.39pm on 9 November to reports of an injured person in Sidcup Bypass, BR7.
“We sent ambulance crews, paramedics in fast response cars, an advanced paramedic and an incident response officer to the scene.
“We also dispatched a trauma team in a car from London’s Air Ambulance.
“We treated two children at the scene, one of whom was seriously injured.
“Both were taken to hospital.”
A Metropolitan Police spokesperson said: “Police were called at 17:44hrs on Sunday, 9 November to Sidcup By-Pass, Chislehurst to reports a child had been injured by a collapsing wall.
“Officers attended and paramedics were treating a five-year-old boy.
“He was taken to hospital by the London Air Ambulance, where he remains in a life-threatening condition.
“A six-year-old girl was also injured and has since been discharged from hospital.
“Enquiries are ongoing.”
