Six fire engines and around 40 firefighters were called to a fire and explosion on Galpin's Road in Thornton Heath.
A terraced house collapsed and neighbouring properties were also damaged. Firefighters rescued four people. Three people were taken to hospital by London Ambulance Service crews and a further person was treated at the scene.
Deputy Assistant Commissioner Richard Welch said: "Firefighters carried out a search of the property and sadly, a child was pronounced dead at the scene.
"Our thoughts are with their family, friends and the local community at this time.
"The welfare of our staff involved in this incident is very important to us and all of them will be offered counselling as a priority."
Around 40 properties were evacuated as a precaution and a 50-metre cordon was put in place.
The Brigade's 999 Control Officers took 15 calls to the incident.
The Brigade was called at 0708 and the incident was under control by 1458. Fire crews from Norbury, Mitcham, West Norwood, Woodside and surrounding fire stations attended the scene. The Brigade's urban search and rescue teams were also on scene. The Brigade worked in partnership with colleagues from the Metropolitan Police, London Ambulance Service and the local council.
The Brigade's fire investigators have begun their investigation into the cause of the explosion and fire. Crews will remain on scene throughout the night.