Parenting groups and unions have criticised changes to the child maintenance system.
Last month, the Child Support Agency (CSA) started closing its cases - meaning separated parents can no longer apply for help to work out financial arrangements to support their children.
Instead, those who cannot agree payments between themselves will be charged to use the new Child Maintenance Service.
Now, the Public and Commercial Service (PCS), has criticised the fact that money meant for the child will be cut by charges from government agencies.
Jayne Craven, the union’s industrial officer, said: “The government believes parents should be able to make their own arrangements when it comes to the care of their children.
“But if they do need help from a government department, they are going to have to pay to use it.
“The money will go to the government coffers and not to the child.
“The feedback that we have had from organisations that deal with children such as Gingerbread is that this could have a fundamental impact on children.”
Charges involved in the new system include a £20 one-off registration fee.
If parties fail to agree a method of payment, non-resident parents will have to pay 20 per cent extra, while the resident parent will be charged four per cent to receive the money.
Ms Craven added: “If you’re on the breadline, this money can make a big difference.”
Falkirk MP Michael Connarty said: “It’s a disaster and it really goes back to the dark ages of responsibility of parents.
“Child maintenance should be part of the tax system where the government ensures that everyone who has a child pays for them.
“What I think we’ll find with this is that these arrangements between parents will break down and more children will be raised in poverty.”
But the government is defending the decision.
Work and Pensions Minister Steve Webb said: “Reform of the old broken system was absolutely necessary.
“The old CSA was just not fit for purpose – it spent £503 million in one year to transfer £1.1bn of maintenance and left more than 50 per cent of children living in separated families with no effective financial arrangement in place.
“The new system is helping more couples to work together to ensure the best outcomes for their children.
“We know children do better when parents work together, even after separation, and I am very encouraged the new child maintenance system is already making this a reality for thousands of families.”
Kathleen Frew from Family Mediation Central Scotland said: “The CSA will be writing to people individually over a long period of time to explain how the changes are going to affect them and what their support options are.
“But the fact is that the CSA is going to seize to exist, and any new applications for help will go to the Child Maintenance Service.
“This will encourage parents right from the start to work out their own family based arrangements.
“While that’s something we’d all encouarge, we have to be realistic and for some people, they are not going to manage.
“Money is always a contentious subject and it may be further complicated by issues between the parents such as contact.
“Family mediation can help, and people won’t need to incur these charges.”