The government says it wants to expand a planned childcare tax credit scheme to include parents who stay at home because they are full-time carers.
A 12-week consultation on the scheme, worth up to £1,200 a child, has begun.
Families with two working parents could be able to claim, and ministers said carers who look after disabled relatives and others would be included.
Chancellor George Osborne said stay-at-home mothers, who had made a "lifestyle choice", would not be eligible.
The government says the overall scheme - set to replace the existing system of employer-supported childcare vouchers - will help 2.5 million families.
'Level playing field'
It is aimed at getting more people back into jobs.
The UK has some of the highest childcare costs in the world, with many people with two or more children saying it does not make financial sense for both parents to work.
The new system is expected be phased in from autumn 2015, with children under five helped in the first year. It will then build up over time to include all children under the age of 12.
Families with two working parents on less than £150,000 each would be able to claim up to £1,200 a year per child.
Critics have said the focus on work will penalise parents who stay at home to look after their children, or are unable to work because they act as full-time "carers" for other adults.
However, extra details released by the government on Monday make it clear that parents who do not work because they are carers will also be eligible.
Carers are defined as those spending at least 35 hours a week looking after someone such as a disabled relative.
But the scheme will not be extended to those who stay at home to bring up their children.
Mr Osborne told BBC Radio 4's World at One programme: "This is help for formal childcare. Obviously it's not for stay-at-home mothers.
"I have huge regard for mothers who want to stay at home and look after their children. That's their lifestyle choice. I want to help those families too. I'm not trying to be exclusive.
"We have a proposal on married couples' tax breaks which I'm going to introduce in the Autumn Statement later this year... that will help stay-at-home mothers."
Lynne Burnham, secretary of Mothers at Home Matter, called for a "level playing field" for all families, with the introduction of a "family allowance" for all households with children under the age of 16.
She added: "It should not be for this government to dictate how a family chooses to care for its children."
Under the proposal, parents will be required to open an online voucher account with a voucher provider and have their payments topped up by the government.
For every 80p families pay in, the government will put in 20p, up to the annual limit of £1,200.
The vouchers will be valid for any Ofsted-regulated childcare in England and equivalent bodies in Scotland, Wales and Northern Ireland.
Half of the funding for the new scheme will come from the abolition of the previous system of employer-supported childcare vouchers - which is provided by only about 5% of employers - and in part by funding switched from elsewhere in Whitehall.
A separate scheme will provide funding for parents who claim universal credit. It will see the state cover up to 85% of their childcare costs, up from 70% at present.
For Labour, shadow children's minister Sharon Hodgson said: "Only David Cameron's government could be so out of touch that they expect families to be grateful for help with childcare in 2015 when they've already seen costs spiralling and support taken away."