Thousands of young people may be "slipping through the net" because adults do not spot the warning signs of mental health problems, experts warn.
MindEd, a new website, backed by groups including the Royal College of Paediatrics and Child Health, is being launched to raise awareness.
A survey of 2,100 adults found a third were unsure of signs of depression in children.
More than 850,000 children in the UK have a mental health problem.
The survey, carried out on behalf of the child and adolescent mental health groups behind MindEd, also found half of those questioned would be worried about saying anything if they did suspect there was a problem, for fear of being mistaken.
Two-thirds would back extra government investment in children's mental health services to equip professionals with the skills to identify and treat these children earlier.
'Still a stigma'
Writing in a Scrubbing Up for the BBC News website, Dr Raphael Kelvin, child psychiatrist and clinical lead for the MindEd programme, said the survey also looked at other ways of helping children.
"When we polled the public on the issue, 69% said they supported the notion that every school should have a dedicated member of staff on site for children to approach about mental health and well-being issues."
The MindEd website, funded by the Department of Health, is aimed at helping adults who work with children, including teachers, sports coaches and social workers, to spot the signs of a potential problem.
Dr Kelvin added: "Half of all diagnosable mental health conditions start before the age of 14 and 75% by the age of 21, so identifying children at the earliest opportunity is crucial in setting them on the best path in life.
"Investing in early intervention is crucial - not doing so comes at a high price for those battling a mental health condition, and also costs the economy vast sums of money in lost education, training, jobs, and often, through crime."
He said: "It's clear from these results that there's still stigma attached to mental health with 51% of adults admitting fear of approaching the issue.
"It's also clear that many adults are not confident in being able to spot the signs of ill mental health in children and many are turning to other adults - family, friends and teachers - for help and advice.
"So it's vital that people know what to look out for so they can address the issue before it worsens and that's where MindEd can help."
Care Minister Norman Lamb said: "Spotting the signs of mental health problems early in children and young people is essential to prevent problems from escalating and continuing into adulthood.
"That's why we have invested £3m in MindEd - so that people working with children, from teachers to dinner ladies and sports coaches to Scouts leaders, can recognise when a child needs help and make sure they get it."