Under the £350m Conservative plans, the amount of state-subsidised childcare for three and four year-olds would be doubled to 30 hours a week reports BBC.
Labour said it was “another unfunded announcement”, accusing the Conservatives of “desperation”.
BBC political correspondent Carole Walker said Mr Cameron was attempting to switch the focus of the campaign to one of his party’s key announcement after several days dominated by warnings about the SNP and a future Labour government.
The party is likely to face questions about how it would ensure sufficient childcare places are available, she added.
Currently, all three and four-year olds in England are entitled to 570 hours of free early education or childcare a year, which works out as 15 hours each week for 38 weeks of the year.
The Conservatives said the 30-hour offer from 2017, announced a week ago, would result in more than 600,000 extra 15-hour free childcare places every year.
They said the proposal would be funded by reducing tax relief on pension contributions.
Labour has pledged 25 hours of free childcare a week and the Liberal Democrats 20 hours, although both parties also plan to extend the offer to younger children.
Mr Cameron will say his government inherited a “shocking” situation, “where couples were spending as much on childcare as one of them took home in earnings”.
He will add that “for many second earners, work didn’t pay because the cost of childcare was so high”.
A Conservative government would expand on the changes made in the last Parliament, Mr Cameron will say.
But Labour’s shadow education secretary Tristram Hunt said: “Hard-working families will not be fooled by the £600m gap in funding for this policy, as announced last week.”
He said Labour had a “better plan” including a “guarantee of access” to childcare between 08:00 and 18:00 for primary age children.