Emmanuel and Emmanuella were last seen playing in the front garden of their address in Cowley Road SW9 at around 6pm before a relative went to check on them an hour later and discovered they were not there.
Officers were alerted and are now out searching the local area with the assistance of the National Police Air Service.
Family and a number of local people are also assisting with the search.
Emmanuella is believed to be wearing a pink top and bottoms with pink shoes and has her hair in braids.
Emmanuel is wearing a blue long sleeved top with blue bottoms and blue shoes.
Their family are desperately worried and officers are concerned for their welfare.
Anyone who sees them is urged to call 999 immediately ref CAD 6803/7 May.
