The Lib Dems say they would cut class sizes for 5-7 year olds
The Liberal Democrats say they would enable English infant schools to reduce class sizes to 15 - half the current maximum - by training more teachers.
They say they would fund this by ending the Child Trust Fund, which gives every child £250 when they are born, and by redirecting other public spending.
Leader Nick Clegg said education would be his party's biggest financial priority in its election manifesto.
The policy would require up to an extra 38,000 teachers to be trained.
The party would allow all infant schools that want to to reduce class sizes for five to seven-year-olds to 15 - it made a similar pledge in 2005, to reduce class sizes to 20 by scrapping the Child Trust Fund.
Under the latest plans, schools which already have smaller than average classes - but still of more than 15 pupils - could take on an extra teacher to float between different age groups, offering help to colleagues.
This could include some one-to-one tuition for pupils with behavioural problems of learning difficulties.
A party spokesman said the policy was "flexible" and would particularly help schools in more difficult areas.
It would not involve getting rid of teaching assistants, he added.
Mr Clegg said cutting class sizes in early years would cost about £500m - the amount currently allocated to the Child Trust Fund.
"There is no point giving someone £250 at the age of 18 if you haven't done what we need to to give young children ... the start they need in school," he told BBC Radio 4's Today programme.
"At a time when we are pouring hundreds of billions of pounds into the pockets of bankers, to find half a billion pounds to give all our children the start they need in life seems to me to be a sensible call."
Among proposals published on Thursday, after what the party describes as its biggest review of education for ten years, are reducing class sizes and paying more to those schools dealing with children from difficult backgrounds and poor families.
Mr Clegg told the BBC it would give them the same chances as children going to fee-paying schools and would allow teachers to give one-to-one tuition and Saturday morning classes.
He said children from disadvantaged children benefited from a "lift" early on.
He told the BBC: "All the evidence shows that if you are a bright but poor child you will be overtaken in school by the age of six or seven by a less bright but wealthier child and after that age the gap opens up ever wider. That is wrong. That has to change."
Christine Blower, acting general secretary of the National Union of Teachers, said: "The emphasis on early years in education is thoroughly welcome.
"All research shows that it is through intervention in the first few years of a child's education that the greatest likelihood of success in schools is fostered."