The Government will need to find pound;85 billion over the next 10 years if its plans for children's services are to succeed, ministers were warned this week.
This would double the pound;3.5bn a year spent on projects such as Sure Start, extended schools and other initiatives.
The Government should also take on the burden of most of the pound;3bn being spent a year by parents on childcare.
Anne Longfield, chief executive of 4Children, formerly the Kids' Club Network, told a conference that the Government should be in no doubt that implementing the proposals "will cost serious money".
Ms Longfield was responding to a speech by Charles Clarke, the Education Secretary, who outlined plans for "wraparound" childcare, which could see primary schools opening for up to 10 hours a day.
She said: "We are talking about many billions. If we are serious about changing the balance in childcare and education we need investment and intervention to make sure that the services are there.
"We need to decide what value we place on childhood in this country."
She added that pound;85bn would amount to spending 1 per cent of the UK's gross domestic product on childcare and early years - half of what Denmark spends.
Mr Clarke told 300 delegates attending the Every Childhood Matters conference in London that schools would become "community hubs" offering "educare", a combination of early-years provision and health services, which would be available all year round. They would be paid for by parents using tax credits.
Mr Clarke said it would take time for all schools to provide extended services, but he said: "We want it to happen and we will be providing practical help, training, capital funding as well as some revenue support to help roll out the strategy across the country and to make a reality of a childcare guarantee for parents."
A report published by 4Children to coincide with the conference showed that while schools were in favour of opening from 8am to 6pm, many were concerned that they would be unable to implement the changes because of a lack of suitable premises, funding and staff.
A third of primary schools still do not offer out-of-school services despite parental demand.
Mr Clarke's proposals were attacked by Emma Hutchinson, of the west London charity Music House for Children, which offers out-of-school music, singing and drama clubs for children.
She told him that extended schools were little more than "boarding schools without beds", and that the nation was storing up problems by consigning young children to school for up to 10 hours a day.
She said: "I believe that in the future this could cause psychological issues for some children who are kept in the same place for long periods.
"The Government should be working instead towards making sure parents are encouraged to be at home with the children. This does nothing for family life."
David Hart, general secretary of the National Association of Head Teachers, said extended schools would eventually become the norm but "they come with a price tag that Government and consumers must deliver".