Working parents would receive 10 hours of free childcare a week from the end of paid parental leave, as would all families with two to three-year-olds.
Childcare for three to four-year-olds would increase from 10 to 15 hours and also apply to all families.
Other parties have also focused on more free care for three to four-year-olds.
Families with children of this age are currently allowed 570 hours of free early education or childcare per year, usually taken as 15 hours a week for 38 weeks.
Free childcare for two to three-year-olds is only available in poorer areas that are part of the Welsh Government's Flying Start programme at the moment and is not provided for children under two.
Lib Dem education spokesman Aled Roberts said: "There is a black hole in government support for families from the end of parental leave to when their child gets a subsidised place.
"Our offer is unique; we are the only party offering free childcare to help parents when their parental leave ends."
The Lib Dem childcare plans would apply for 38 weeks a year, although the party said it wanted to expand it to 48 weeks "in the longer term".
Labour and the Conservatives have said they would increase free childcare for three to four-year-olds from 10 to 30 hours a week for working parents.
Labour's proposals cover 48 weeks of the year, while the Conservatives have not yet specified how much of the year their pledge applies to.
Plaid Cymru would raise the number of hours for three to four-year-olds to 30 hours a week for all families, for 39 weeks a year.
UKIP has said the main problem is the limited supply of childcare, something it would tackle by allowing child minders to work without needing to get regulatory approval.
The Liberal Democrats have announced plans for free childcare for children from nine months old, if the party wins power in May's assembly election.