One in three children in Wales live below poverty line
Children born in Wales this century are more likely to be living below the poverty line than their peers in the rest of the UK.
Researchers found one in three Welsh seven-year-olds live in a family with less than 60% of the UK's average household income
But most youngsters are thought to be in excellent health with many friends.
The assembly government said tackling child poverty is a "top priority".
The research was carried out by the Institute of Education at the University of London and involved a survey of 14,000 children born between 2000 and 2002 from across the UK.
Overall it found that while the millennium generation of children in Wales do experience a relatively high poverty rate, their parents believe their lives get off to a healthy start.
In Scotland, which has the lowest poverty rate of all four countries, just over one in four of the millennium families (26%) had such a low income.
However, some English regions had higher poverty rates than Wales: the North East (40%), other northern regions (35%) and London (36%).
Meanwhile, nearly one in four children in Wales are being brought up by a single mother.
Just over half (51%) of Welsh seven-year-olds are in families where their mother and father are married, compared with 61% in Northern Ireland, 55% in England and 53% in Scotland.
The Millennium Cohort Study (MCS) was set up by the Economic and Social Research Council and co-funded by government to use data to compare all aspects of the lives of children across the UK.
It follows the lives of a sample of babies born between 1 September 2000 and 31 August 2001 in England and Wales, and between 22 November 2000 and 11 January 2002 in Scotland and Northern Ireland.
The MCS also found that youngsters in Wales and Northern Ireland are the most likely to say they have "lots" of friends.
More than two thirds (68%) claim to have many friends, compared with a UK average of 63%.
And almost two-thirds (63%) of the 2,000 Welsh mothers surveyed described their child's health as excellent - a slightly higher proportion than in England and Northern Ireland (both 59%).
The UK Government has pledged to end child poverty by 2020.
Save the Children last week issued a warning over new plans to cap the maximum amount of benefits that any single family can claim at about £26,000 - the same amount that an average family gets from work.
"Ensuring that the 3.9 million children who still live in poverty in this country are not made worse off at this time of spending cuts must be the priority," said Save the Children's Sally Copley.
"Any savings from this move must be ploughed back into giving more assistance to the poorest children and ensuring families are better off when a parent takes a job or increases their hours."
Responding to the findings of the MCS, Deputy Minister for Children, Huw Lewis, said politicians were exploring new ways of addressing child poverty in Wales.
He said: "Tackling child poverty and improving the life chances of children and young people from low income families is at the top of our list of priorities.
"We were the first of the UK countries to introduce new legislation to add weight to our action to tackle child poverty and recently consulted on our Child Poverty Strategy."
He said that a range of cross-government policies had been outlined with the aim of reducing the inequalities that currently exist.
He added: "We are also looking at new and innovative ways of working with families whilst treating people as individuals. A one size fits all solution simply won't work.
"This is why I recently announced our Families First Pioneer areas. Wrexham and Rhondda Cynon Taf local authorities have been selected to lead the way in demonstrating innovative ways of working and best practice models in their efforts to support families."