Longfield, a member of the Strategy Unit team that developed the policy, estimates charities will fill 20 per cent of the Government's target of 1,600 children centres in the most deprived areas.
The centres will provide a single place for tackling all children's issues.
Longfield predicts the integrated approach will force children's charities to work more closely with the Government and occupy less of a niche.
"It presents an enormous challenge to the voluntary sector to look realistically and honestly at what they do," she said. "More and more funding will come through joined-up thinking, and if charities want to receive it, they are going to have to fit in. People are going to have to talk to each other much more and come up with interlinked programmes."
Longfield said a more holistic approach to childcare was something charities had long demanded and having closer ties with the Government was a price worth paying. "It's very difficult to rattle around different government departments," she said. "This places children at the centre and the services around them."
Children's Minister Margaret Hodge last week announced the locations of the first 32 centres. "Children centres provide a single focus for five key services - early education, childcare, health, family support and help into employment," she said.
The National Day Nurseries Association's five early years centres, which are being built at a cost of £5 million, will be converted into children centres.
BAN ON SMACKING
An alliance of more than 350 organisations, many from the voluntary sector, is celebrating the decision by two parliamentary committees to call on the Government to change its current stance and ban parents and carers from smacking children.
Children are Unbeatable! has welcomed recommendations by the Health Select Committee and the Committee on Human Rights, which say that the 140-year-old law which permits smacking can be used to hide abuse.
Several MPs have taken up the alliance's cause and put forward an early day motion proposing that the "reasonable chastisement" defence - originally enacted so that men could beat their servants, wives and children - should be scrapped.
However, the move to ban smacking has been criticised by some groups in the sector. Family and Youth Concern feared that the proposal would be used to criminalise parents. The Conservatives have also attacked it, saying it will curb parents' rights.
Members of the alliance include most of the major children's charities, Relate, Scope and St John's Ambulance.