According to UNICEF’s Report Card 10, the United States has the highest percentage of children living in poverty.
The definition of poverty is as follows: “a child is deemed to be living in relative poverty if he or she is growing up in a household where disposable income, when adjusted for family size and composition, is less than 50% of the median disposable household income for the country concerned.”
The comparison in the chart is for the 20 OECD countries with annual per capita incomes of more
than $31,000. (When the comparison is expanded to the 35 economically advanced countries, the United States falls to number 2, behind Romania.)
To be honest, the United States does not have the highest (or even second highest) child poverty rate based on “market income” (before taxes and transfers). But it certainly does based on “disposable income” (after taxes and transfers have been taken into consideration).