Statistics say that there are 600,000 children living in poverty in London.
From the BBC, we learn more of what the charity hopes to do for London's children.
The fund will spend £1.5m next year and aims to have an annual turnover of up to £20m by 2013.
"Over 600,000 children live below the poverty line and London is home to some of the most deprived boroughs in the country," said Mr Johnson.
"Like Robin Hood we want to draw riches from wealth creators to give life-changing support to the poorest Londoners."
The charity aims to invest in projects with "proven results" in tackling child poverty.
The charity's chief executive Chris Robinson said: "We have worked hard to identify what is really needed and the evidence suggests it is to back proven delivery, and to coordinate and connect services rather than suggest there is some miracle cure."
Among them are three schemes in Shoreditch, east London, which will benefit from an £8m cash injection in 2010 before being expanded in 2013.