In briefing the £600 million move, expected to result in a consequential £60m windfall for the Scottish Government, LibDem HQ signalled the Tories would announce at their conference a £600m quid pro quo plan to give all married couples across Britain a tax break, a proposal the Deputy Prime Minister has dismissed as "Edwardian drivel".
A senior LibDem source said it was "a straight up and down deal" between the Coalition partners.
At present, children in Scotland and England can only get free school meals if their parents receive welfare benefits.
In what is being billed as his most personal leader's speech to date, Mr Clegg will say that being a father-of-three has reinforced his belief that education is the key to improving social mobility.
He will stress his prime focus in the Coalition has been "using the muscle of the state to create a level playing field when it counts most: when boys and girls are still forming their views, their characters, their hopes and their fears.
He will say: "That's why I am delighted to tell you we are now going to provide free school meals for all children in infant school".
From September 2014, every child aged five to seven south of the Border will get a "healthy lunch every day", saving families £437 a year per child.
The party leader will stress: "My ambition is to provide free school meals for all primary school children; another reason we want to get into government again next time round."
A senior aide to the leader said there would be Barnett consequentials for Scotland - to be announced in Chancellor George Osborne's autumn statement - and noted: "It's up to the Scottish Government what they do with the money but we would encourage them to spend it on this."
The move has received union backing, with one urging Labour to commit to an improved scheme.
GMB national secretary Brian Strutton said "GMB represents most school kitchen staff and we have been campaigning for universal free school meals for several years.
"The announcement is a welcome first step but to be effective free school meals must apply throughout primary schooling. GMB hopes this will be a Labour commitment for the next election."
In his speech, Mr Clegg will contrast his party's priority to help children with the Tories.
They, he will say, "have made it clear their priority is to help some families over others with a tax break for married couples; a tax break for some funded through the taxes of others. We will help all families in these tough times, not just the kind we like best, by helping their young children get the best possible start in life."
On the Coalition, the LibDem leader will tell his party to "feel proud", saying: "The recovery wouldn't be happening without us. We have made sure the deficit is being cut at the right pace.
"We were the ones who said you don't just get growth by cutting red tape, government also needs to invest in things: infrastructure; apprenticeships; regional growth."
Mr Clegg will list his party's achievements in Government, from helping the poorest children through the Pupil Premium in England and a boost to apprenticeships across the UK to equal marriage and taking three million people out of income tax and equal marriage.
He will tell delegates: "The Liberal Democrats don't want to go back to the Opposition benches because we aren't done yet."
He will say the "clapped out politics" of single party rule threaten to undo the sacrifices people made through the downturn, noting: "Only the Liberal Democrats can finish the job and finish it in a way that is fair."
The DPM will argue that the LibDems are "not just some subset of the Labour or Tory parties" but have distinct liberal values.