The report from the United Nations Children's Fund and the World Bank found that the pandemic led to the abolition of extreme poverty for 30 million fewer children than was previously predicted.
As a result, around one in six children still live on less than €2 per day, according to the report.
"Compounding crises, from the impacts of Covid-19, conflict, climate change and economic shocks, have stalled progress, and left millions of children in extreme poverty," UNICEF Executive Director Catherine Russell said in a statement.
The report's findings throw a spanner in the works of the UN's ambitious goal to eradicate extreme child poverty by 2030.
"A world where 333 million children live in extreme poverty - deprived not only of basic needs but also dignity, opportunity or hope - is simply intolerable," World Bank Global Director for Poverty and Equity, Luis-Felipe Lopez-Calva, said in a statement.
40% of children in sub-Saharan Africa live in extreme poverty
The report found that 40% of children in sub-Saharan Africa still live in extreme poverty - the highest percentage of anywhere in the world.
A series of factors including rapid population growth, Covid-19 and climate-related disasters have exacerbated extreme child poverty in sub-Saharan Africa in recent years, even as all other regions in the world have seen a steady decline.
The World Bank and UNICEF called on countries to prioritise tackling child poverty and to enact a range of measures including the expansion of universal child benefits programmes.
"We cannot fail these children now," said Ms Russell.
"Ending child poverty is a policy choice."