The mother daughter duo sat in on the "high level" meeting at the House of Lords where figureheads discussed how to improve education in Sierra Leone, posing for photographs and chatting with participants afterwards.
The Duchess of York, who is the organisation's founder patron, wore a black, high neck dress with opaque tights and dainty black pumps.
Fergie, 59, added a pop of colour with a bold blue blazer which had large gold buttons on the waist and cuffs.
The famously fiery red head let her locks hang loose round her shoulders and accessorised with slim bracelets and tiny hoop earrings.
Beatrice, who acts as a global ambassador for Street Child along with sister Eugenie, echoed her mother's sartorial choices with her own black, long sleeved dress, thick tights and black loafers.
The 30-year-old granddaughter of Queen Elizabeth bundled up against the British cold in a tailored teal coat with wide lapels and huge pockets.
Last April, Street Child merged with international development charity Children in Crisis which Sarah founded in 1993 to improve education for young people living in poverty and areas of conflict.
Children's causes are close to the Duchess' heart, and Fergie has previously spoken about how her charity work has "kept my feet on the ground" and reminds her how lucky her own kids are.
"Eugenie, as you know, is disabled with 12 inch metal rods down her back. Beatrice is dyslexic with special needs at school. And yet, they had an education, so why isn't every other child allowed that same luxury? And why is it a luxury?" she said in Press Association interview last November.
Watch this video to get Fergie's response to the rumoured Meghan vs. Kate feud