Analysis by the Institute for Fiscal Studies, into levels of government spending on children between 2000 and 2020, commissioned by Longfield, found that that levels of government spending on children have been broadly maintained over the last 20 years.
Last year, total spending on children from the main government departments which fund services for children - excluding healthcare where data is limited - was more than £120bn or £10,000 per child under 18. This is 42 per cent higher in real terms than it was in 2000/01, although 10 per cent below the high point of £11,300 in 2010/11.
However, the analysis found a number of "deeply concerning trends", with mainstream and acute services, such as mainstream education and support for children in care, increasingly protected in recent years at the expense of targeted preventative services.
Almost half of spending on children's services now goes on the 73,000 children in the care system, while the other half has to cover the remaining 11.7 million children in England.
Altogether, it was found that 72 per cent of children's services budgets go towards helping families in "severe need".
The report found that there has been a "significant reorientation" of spending in recent years towards statutory help for children in crisis, while overall children's services spending has been largely frozen since 2009/10.
Spending on preventative support, such as Sure Start children's centres and youth services has consequently been cut by around 60 per cent in real-terms between 2009/10 and 2016/17.
The study also found growing pressure on "high needs" education budgets, with more children than ever now attending specialist high-needs institutions. The number of pupils in maintained special schools increased by 25 per cent between 2007 and 2017, far outstripping overall pupil growth and putting pressure on overall schools funding.
Longfield said: "This analysis shows that while overall public spending on children has been broadly maintained over the last 20 years, millions of vulnerable children who are not entitled to statutory support will be missing out because of the huge cost of helping a small number of children who are in crisis.
"While every child should receive the support they need, the economic and social costs of this current strategy are unsustainable. The cost to the state is ultimately greater than it should be and the cost to those vulnerable children missing out on support will last a lifetime. Every day we are seeing the consequences of helping children too late - in pressures on the family courts system, special schools and the care system and in the spiralling numbers of school exclusions and the consequent increase in younger and younger children linked to violent street gangs.
"I hope this analysis will help to move the debate on from one simply about the amount we spend on children, to a debate about how we spend it. Next year's Spending Review offers an opportunity to step in and support these children falling through the gaps, avoiding government silos and designing cross-departmental services built around a clear identification of the unmet needs of kids."
Richard Watts, chair of the Local Government Association's children and young people board, said: "The LGA has warned for some time that the current situation facing children's services is unsustainable. Last year saw the biggest annual increase in children in care numbers since 2010, and councils are now starting more than 500 child protection investigations every day on average.
"Councils across the country have worked incredibly hard to protect funding for the most vulnerable in our communities despite significant and ongoing government funding cuts, and continue to provide essential help and support for thousands of children and families every day.
"However, this report paints a stark picture of the reality facing councils, who cannot keep providing this standard of support without being forced to take difficult decisions and cut back on early intervention services which help to prevent children entering the care system in the first place.
"It also highlights the growing pressures around supporting children with special educational needs and disabilities, which reinforces our call for an urgent review of how this provision is funded.
"Children's services are being pushed to the brink, and face a funding gap of almost £2bn by 2020 just to maintain current service levels."