The childcare sector is so worried about vulnerable children losing access to two days a week of early education it is urging senators to reject an overhaul of subsidies if the Turnbull government does not agree to changes.
The Senate is debating the government's childcare plan and a vote is expected on Thursday night.
The sector wants the minimum subsidised care guaranteed to low-income families lifted from 12 to 15 hours a week.
But that looks unlikely to happen after South Australian senator Nick Xenophon and his team decided to support the government's position.
Under the planned overhaul, childcare subsidies will be rolled into a single means- and activity-tested payment.
Both parents have to work, volunteer or study for at least eight hours a fortnight to get subsidised care.
Early Childhood Australia says with the rise of insecure and irregular work it would not be unusual for a lower-income family to have their shifts reduced for a fortnight.
"You've either got to decide to pull your children out of childcare or pay the full cost of childcare with no subsidy for that two weeks," chief executive Sam Page told AAP.
"When you're on a combined income of $70,000 and you've just lost work, what hope do you have to cover that without a subsidy?"
Senator Xenophon said his team would support the government on the activity test.
"You can extend the activity by reading to your child at the preschool, volunteering, a whole range of matters that will be set out in debate," he told AAP.
Ms Page said not everybody would be able to volunteer just to keep their kids in care, especially if they were looking for work.
Labor says it is unthinkable to spend $1.6 billion extra on childcare subsidies but leave behind the most vulnerable children.
Government figures show about one in five families earning less than $65,000 a year will lose subsidies because they do not meet the activity test.
"It is not right to punish vulnerable children for the decisions that their parents make," Labor childcare spokeswoman Kate Ellis said.
"The effects of that would be to lock in welfare dependency for generations."
But Labor will not say if it will oppose the bill if it cannot secure its changes.
The Greens have said they will oppose the whole package if the minimum hours of subsidised care are not increased.
The government needs support from nine of the 10 crossbenchers.
Derryn Hinch and Liberal Democrat David Leyonhjelm have secured a promise from the government to scrap childcare subsidies for families earning more than $350,000 instead of covering 20 per cent of their fees up to $10,000 a year.
Senator Hinch says will support the government as long as it makes that change but AAP understands Senator Leyonhjelm will not necessarily vote for the bill.
Former Liberal senator Cory Bernardi has signalled he is unlikely to support the childcare package.
Education Minister Simon Birmingham says negotiations are continuing but AAP understands the government is quietly confident of success.
Social Services Minister Christian Porter says the government believes keeping a minimum 12 hours of care strikes the right balance between helping parents who are looking to work more and making the package financially sustainable.
"Failing to support the reforms would mean we fail to focus our resources on the hardest-working families in Australia on the lowest incomes. That is where childcare is critical," he told parliament.
The legislation includes protections for people who work irregular shifts so their activity test would be based on average hours worked over three months instead of the actual hours each fortnight.
