Speaking during a fringe meeting at the annual Conservative party conference in Manchester, Javed Khan said the difficult financial climate, combined with growing demand, means it is time for "radical solutions".
Pointing to an ongoing partnership between Barnardo's and Newport Council in Wales, he said the voluntary sector should be considered by local authorities to contribute to both the design and delivery of children's services.
"Children's services that are currently provided need to undergo a fundamental change," Khan said.
"A major overhaul is required to be able to meet the demand that currently exists and is going to increase further," he said.
"I think the future definition of children's services has got to be far broader - it has to include the voluntary sector within that and other partners possibly as well.
"Invite us in at the beginning [of the service design process], let us work with you from all of our experience."
Khan conceded that the proposal may be controversial, and indicated that if it was a choice, between design and delivery of services, Barnardo's would prefer to forego delivery in favour of establishing provision that works effectively.
"For some people [co-design and delivery of children's services alongside charities] is a bit too radical, with blurred lines in terms of conflict over if you are designing the solution how can you be the deliverer?
"That doesn't matter to us, we would rather work with you to develop the right solution for vulnerable children and walk away from the delivery if we have to."
The partnership between Barnardo's and Newport Council, now entering its fourth year, has seen the organisations combine on the design and delivery of services - specifically around integrated family support. The service, which has around 100 staff, has a Barnardo's employee as head of service.
"Half of the staff within the service are Barnardo's and half from Newport Council, but if you went into a room and talked to them you wouldn't know which was which, and actually it doesn't really matter because beneficiaries just want good-quality services at their fingertips, when they need them," Khan said.
Barnardo's had been due to work alongside Norfolk County Council on a jointly run looked-after children's service, an arrangement that was announced by former Education Secretary Nicky Morgan in July 2016, who described it as part of the "widest-reaching reforms to children's social care and social work in a generation".
However, the plans failed to attract backing from the DfE's social care innovation fund, which turned down a £7m grant application earlier this year.
In June the organisations said they would start a separate joint project to reduce the number of children in care by providing intensive support to at-risk families, and helping return children home from care, whenever it is "safe and in their best interests".
Khan said he is talking to local authority commissioners across the UK about further partnerships, adding that there is "a lot of interest".
"We are in deep conversation with a number of other places on how we can create a bespoke solution to their particular challenge, working in partnership with us, on the condition that they have us at the table where we dual-produce the solution, right from the start, rather than as an afterthought," he said.
"I think that's quite radical, and radical solutions are what's required if we are ever going to get to grips with the enormous challenge (of increasing demand)."
Khan also criticised the commissioning process for children's services - the fact that many service provision contracts are for one or two years.
"This short-termism that has appeared, it doesn't wash," he said.
"The one thing that we have learned from all our years delivering large-scale children's services is that the most vulnerable need long-term sustained interventions to help turn around their lives. Short-term sticking plasters don't work - it is throwing money at something that isn't effective."
He added that the tender process itself can often cost organisations significant amounts of money to bid for a contract, but only one winner emerges.
"All the others have wasted their valuable resource in the competition," he said.
"I have nothing against competition, but this system is not sustainable. Demand [on children's services] is increasing beyond imagination, and resources are being wasted. Something better has got to be done. A better way has to be found, otherwise, we are all going to regret it."
