The US has one of the highest rates of relative child poverty among the world's wealthiest countries, according to a report by the UN.
Higher government spending on families is needed, the UN says
The US, which is second only to Mexico in the UN children's agency report, is nonetheless one of few countries to see a recent decline in child poverty.
In total, Unicef says up to 50 million children are living in poverty in rich nations and the figure is rising.
Children in Nordic countries are best off, due to higher social spending.
Unicef looked at 24 of the 30 states in the Organisation for Economic Cooperation and Development (OECD) - a Paris-based group of the world's wealthiest nations.
The figures refer to relative poverty defined as households with income per head below 50% of the national average.
Its Child Poverty in Rich Countries report found that the number of children living in poverty had risen in 17 of those countries over the past decade.
Mexico comes bottom of the table with a figure of 28%.
"No matter which of the commonly used poverty measures is applied, the situation of children is seen to have deteriorated over the last decade," the report says.
Even in the few countries where deprivation is declining the rate can remain high - as is the case in the US, where about 22% of those aged under 18 are still living in relative poverty.
Similarly, the UK still has 15% of the child population below the poverty line despite government campaigns which have led to a 10% drop.
Unicef regional director Philip O'Brien stressed that the figures were relative to the average household income of the countries involved, rather than their national wealth.
"The child living in poverty in the US is clearly not as badly off as the child in Mexico," he said.
Top of the table are Denmark and Finland, where child poverty levels are less than 3%, while Norway and Sweden follow close behind.
Unicef praised the Nordic nations for their social spending on families.
"Higher government spending on family and social benefits is very clearly associated with a lower level of child poverty," said Mr O'Brien.
He said market forces could not on their own lift children out of poverty and urged direct intervention through greater government spending.
Only the US, the UK, Australia and Norway have had significant drops in child deprivation, according to the figures supplied to the OECD over the past 15 years.