The government has failed to reverse the decline of children's rights in the UK, which is still 'demonising' young people.
The UK's four children's commissioners, in their first combined report, say children's rights in the country have worsened over the last five years.
Last week the government announced plans to increase the number of children prosecuted for carrying knives following a spate of fatal stabbings involving teenagers.
Other areas of concern flagged up at the time of their last report have remained unchanged, the commissioners add.
The hard-hitting report says "serious concerns" remain over significant breaches of the UN Convention of the Rights of the Child (UNCRC).
It criticises the government and devolved administrations for failing to tackle child poverty and the persistent inequalities experienced by children.
Kathleen Marshall, the commissioner for children in Scotland, demanded on Monday the UK fully implements the UNCRC.
"We have highlighted areas that remain a concern, including significant differences in juvenile justice in some parts of the UK and the public's attitudes towards children and young people," Ms Marshall said.
Commenting on the report, Howard League for Penal Reform director Frances Cook called for the use of physical restraint on children to be banned, as well as the recognition that juvenile custody was being overused.
"The children's commissioners have come out fighting the corner for children, who remain disenfranchised and insufficiently heard in our society," Ms Cook explained.
"Children in custody should be treated the same as any other children."
Lib Dem children's spokesman David Laws said: "This report confirms that Britain is failing millions of its children, who grow up in poverty, poor housing and in unstable family environments.
"For all the government's pledges and bluster, Britain is still one of the worst countries in the developed world in which to be a child."