The Children's Commissioner obtained data from 48 of England's 60 child and adolescent mental health service trusts, and discovered 28% of child referrals were denied specialist treatment - mostly on the grounds that their illness was not serious enough.
A review by the Children's Commissioner also found that 13% with life-threatening conditions were not allowed specialist support.
It also found that those who secured treatment faced lengthy delays, with an average waiting time of more than 100 days. NHS England said it was "clearly the case" that services need to expand.
Children's Commissioner Anne Longfield told BBC Radio 4's Today programme that she had heard from a "constant stream of children, parents and professionals" about their inability to get help when they really need it.
They go to their GP who refers them to specialists, but the specialists then say their conditions are not serious enough, she said. "I don't yet know quite why they are being turned away, but certainly being turned away or put on a waiting list for up to six months is clearly playing Russian roulette with their health," she added.
The average waiting time for those accepted for support ranged from 14 days in a trust in north-west England to 200 days at one in the West Midlands.
More than a third of trusts, around 35%, said they would restrict access to services for children who missed appointments. Sarah Brennan, from mental health charity Young Minds, said: "Services have been cut and young people had no where to go."They are then more ill when they get help so services have become overwhelmed... Six months for a young person is huge and in that time most young people are becoming more ill."
James Morris, the Conservative MP who is chair of the all-party group on mental health, acknowledged that problems had been building up in the system over many years and a "fundamental transformation" was required.
"We do need to move towards a more compassionate system for children and young people but the transformation is going to take time
In March, the Mental Health Network, which represents mental health trusts, said it had seen "no significant investment" in psychiatric services for children in England. It said it suspects some of the money allocated has been used to support other NHS services instead.
In another survey, launched at the Association of School and College Leaders' (ASCL) annual conference in Birmingham, it was found that over the past five years: 79% of heads saw an increase in self harm or suicidal thoughts among students. 40% reported a big rise in cyber-bullying 53% of those who had referred a pupil to Child and Adolescent Mental Health Services (CAMHS) rated them poor or very poor. Overall, 80% of respondents wanted to see CAMHS expanded in their area.
Children today face an extraordinary range of pressures. They live in a world of enormously high expectations, where new technologies present totally new challenges such as cyber-bullying. There has seldom been a time when specialist mental health care is so badly needed and yet it often appears to be the poor relation of the health service.
Anyone can see that there are major problems with the NHS and it seems a there is a recipe for disaster is being cooked up. It is up to us the public to voice our concerns. If you read this article don’t stay silent, please,