Campaigners are calling for child benefit to be increased on the welfare payment's 60th anniversary.
The Child Poverty Action Group's Make Child Benefit Count campaign is also calling for younger children to be paid at the same rate as the oldest child.
Child benefit is currently £17.45 per week for the first child and £11.70 per week for subsequent children.
The predecessor of child benefit - family allowances - were first paid to parents on 6 August 1946.
The campaign is also being backed by charities End Child Poverty, Save the Children, Citizens Advice, Barnardo's, the TUC and the Family Welfare Association.
As part of its campaign the Child Poverty Action Group (CPAG) is asking people to send an electronic postcard from its website to the Chancellor Gordon Brown.
'Society's commitment'
The charity is also publishing a report that says increasing child benefit and paying it at the same rate for all children would help to support larger families.
CPAG chief executive Kate Green said: "This birthday is time to celebrate the role child benefit plays, showing society's commitment to our children and helping parents make ends meet.
"Child benefit is popular, effective and reaches more children living in poverty than any other benefit or tax credit.
"That's why we're calling on the chancellor to increase child benefit and ensure that younger children get the same rate as the oldest child."
Useful addition
The CPAG's campaign is being launched by one of the first mothers to receive the first family allowances payments.
Edith Wyper, 86 from Edinburgh, also worked as a sub-postmistress and as a result actually paid out many family allowances herself.
She said: "They were useful as an addition to your income and they helped poorer families, where even a shilling was quite a sum.
"I saved it, maybe for six, seven or eight weeks, until I had a lump and then I'd spend that on clothes and shoes and things that my children needed."
Colette Marshall, Save the Children UK Director, said: "Having children doesn't get any cheaper after you've had your first child.
"Receiving £5.75 less a week for a second and any subsequent children is a significant drop in financial support for those families with little money coming in.
"The government has committed to halving child poverty by 2010 and should adopt this policy as a vital step to reaching this target."
There are currently 7.3 million families receiving child benefit for 12.9 million children, and the CPAG claims that paying the same rate for all children would benefit four million families with two or more children.
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