Rosemary was last seen at 17:45hrs on Monday, 14 February in the NW8 area. She has links to Westminster and Hackney.
The children are an eight-year-old girl and a six-year-old boy.
Police Sergeant Darren Bull, said: “We are very keen to hear from anyone who has seen Rosemary Goncalves and the children or knows where they are.
“We need to know that all three are okay, and Rosemary, if you are reading this, please get in touch or go to your nearest police station.”
Anyone who has information that could assist officers should call 101 or Tweet @MetCC quoting reference CAD 5474/14Feb.