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1 July 2015 Last updated at 06:46 BST
Sally Holland said children needed to be protected from the impact of Westminster austerity measures.
She said too many children in Wales were being denied a decent childhood.
Ms Holland and the commissioners for England, Scotland and Northern Ireland said in a report to the United Nations they were alarmed at how welfare changes had increased child poverty.
They said under current policies figures were expected to rise, with 4.7m children projected to be living in poverty by 2020.
Figures released last week showed a third of children in Wales lived in poverty - defined as such when in a household with an income below 60% of the UK average of Â£453 a week.
The charity Barnardo's welcomed the report, and said millions of children were being failed.
The UK government must stop making cuts to benefits or more children will be "pushed into poverty", the children's commissioner for Wales has said.