Mental health services in England will receive £1.25bn in next week's Budget, Deputy Prime Minister Nick Clegg has confirmed.
He said the cash, spent over five years, would mainly go towards helping more than 100,000 young people.
The plan will also see the first waiting-time standards for children and extra specialists in talking therapy.
Labour has previously criticised the government for cutting spending on children's mental health services.
Chancellor George Osborne will set out his spending plans in the Budget on Wednesday.
'An important boost'
The Liberal Democrats say the extra cash will give the NHS the resources to treat more than 100,000 young people by 2020.
It will also be spent supporting pregnant women and new mothers.
The chief executive of NHS England, Simon Stevens, has described the extra funding as "an important boost".
Speaking during a visit to Clock View Hospital in Liverpool, Mr Clegg said: "There would be an outcry if a child with diabetes was left to cope without support or treatment.
"But that's exactly what's been happening with young people's mental health services.
"That's why I am determined to start a seismic shift to revolutionise children's mental health care and end this unacceptable injustice."
He said new mothers had previously struggled with a "second-class mental health service".
"It is terrifying to think that in this day and age some new mothers are having to travel miles for treatment and others are even being separated from their newborn child - this has to stop," he said.
"This funding will make sure they get the treatment and support they need, so they in turn can give their children the best possible start in life."
The funding will help children with conditions such as self-harming, depression and those at risk of committing suicide.
Therapy sessions, family support work, better training for clinicians and the development of help via websites and online apps will be supported by the cash.
Liberal Democrat Treasury Secretary Danny Alexander told the Times newspaper: "A mental health package will be announced in the Budget and there will be a children's component.
"Nick (Clegg) has made mental health a huge thing for our party - it's going to be one of the five key pledges on our manifesto."
He said the money would fund early intervention schemes to stop children developing serious and potentially fatal mental illnesses.
The package is also expected to double the funding for armed forces veterans.
The extra money to support veterans will fund 10 mental health teams to increase help and treatment for servicemen and women.
The BBC's political correspondent Ross Hawkins said the Liberal Democrats would put a commitment to mental health services on the front page of their election manifesto, and had made it a central feature of their pre-Budget negotiations.
Last year, Mr Clegg announced the first waiting-time standards for mental health treatment and £120m pounds to improve services, pledging to "end discrimination against mental health".
In January, Labour accused the government of breaking its promise to make mental health a priority after it was revealed that NHS spending on children's mental health services in England had fallen by more than 6% in real terms since 2010.
Ministers pointed to an extra £7m spent this year on more psychiatric beds.
Child and adolescent mental health services are under increasing pressure, juggling tight resources with increasing demand.
In November, a 16-year-old girl from Devon spent two nights in a police cell as no psychiatric bed was available.
Investigations by BBC News and the online journal Community Care last year revealed an increasing number of children were being treated on adult wards while others were travelling hundreds of miles to get care.
An investigation by the charity Young Minds also found more than half of councils in England had cut or frozen budgets for child and adolescent mental health between 2010-11 and 2014-15.
Mr Osborne is set to deliver his sixth Budget in the Commons on Wednesday - the last before May's general election.
It comes as Business Secretary Vince Cable told party supporters he was proud of the party's record in government - even though he said being part of the coalition had been "tough" for activists.
"Even those of us in the privileged position of being at the centre of government are forced to confront the question, has it been worth it? For the country, absolutely. But for the party? My answer is yes, unambiguously," he told the Lib Dem spring conference in Liverpool.
Setting out how the Lib Dems want to progress the economy from "rescue to recovery to a truly great British industrial renaissance", he said the UK needed a "much stronger skill base".
The Lib Dems planned to "raise" the level of vocational skills training and to "build on our commitment to innovation and science", Mr Cable said.
"We have a vision of a British industrial renaissance in which financial institutions cease to operate like a casino and there is adequate long-term investment - private and public," he added.