It is the third children’s charity in the past 18 months to become part of Street Child, which helps vulnerable children access education in developing countries. Children in Crisis joined in April 2018 and the Lessons for Life Foundation followed suit in June this year.
A Street Child spokesman said the merger came about after Build Africa concluded it could have greater impact by joining another charity.
Build Africa approached Street Child because it had seen the other two charities join its group in recent months, the spokesman said.
Street Child said the merger would enable it strengthen its work in Uganda and begin working in Kenya.
Three of Build Africa’s six staff have joined Street Child’s workforce of about 50 people, with the remaining posts being made redundant.
Julie Randles, who was interim chief executive of Build Africa, left the charity after completing her contract last week, the spokesman said.
Tom Dannatt, who founded Street Child in 2008, remains chief executive of the merged charity.
The spokesman said the Build Africa brand would continue to be used in Uganda and Kenya "in the short term at least".
Three Build Africa trustees will join an enlarged Street Child board as a result of the move, and its staff have moved into Street Child’s offices in south London.
Dannatt said: "The addition of Build Africa to Street Child is a powerful and strategic expansion of our capability to create impact in east Africa in particular, both for the huge numbers of refugee children desperately needing education support in the region and for the most impoverished local populations.
"I am certain, as we are already seeing with Children in Crisis and Lessons for Life, that we will achieve more together."
Andrew White, chair of Build Africa, said: "By becoming part of Street Child we not only have the clear opportunity to create impact beyond the sum of our parts but, as part of a global charity, we effectively extend Build Africa’s impact much more widely across the African continent."