A "recovery plan" put forward this week by Northamptonshire County Council includes proposals to make "radical service reductions and efficiencies" across the authority to "significantly reduce spending".
The council says it needs to make £60-70m of savings in 2018/19.
Service areas under review include children's services, which was rated "requires improvement" by Ofsted in April 2016, and which is expected to be transferred to an independent trust called Children First by April 2019.
Children's social care spending will be particularly scrutinised, with the plan stating that "factors for high spending on high numbers of referrals and high numbers in the care system" will be assessed.
In addition, the plan proposes tighter controls on all spending over £1,000 and introducing a new approval process for spending in children's and adults services.
The plan is to be considered at a cabinet meeting of the Conservative-controlled council on 14 August.
Penny Smith, Unison Northamptonshire branch secretary, said the exact implications of the plan on children's services is unclear but warned that provision for vulnerable children is likely to be affected.
"The council is once again letting the most vulnerable people down and putting their health and wellbeing at risk," she said.
"Local residents will increasingly struggle to receive the most basic of services… and the council is at risk of failing to keep children and vulnerable people safe. This could cost lives."
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Meanwhile, plans to close 21 of the area's 36 libraries - which, also house the council's children's centres - by the end of the month have been shelved. However, Smith said that many staff had already left or been made redundant affecting provision.
Unison is calling on the government to provide emergency funding to pay for basic services - in May, Communities Secretary James Brokenshire appointed two commissioners to oversee the running of the council.
Council leader Matt Golby said: "The new priorities being discussed and agreed by councillors over these next two weeks will be used to assess all spend across council services and identify where savings can be made.
"Of course 70 per cent of the council's spend is through contracts with third party suppliers and a large piece of this work will be to review all of these contracts against the new priorities on a contract by contract basis.
"We will be open and fully transparent with the results of this work which will include painful but necessary decisions."
In April, a government report recommended Northamptonshire County Council be scrapped and replaced with two unitary authorities after the council issued a "section 114 notice" in February, banning any new spending with the exception of safeguarding vulnerable children and adults.