More than a fifth of children referred to mental health services in England have been refused treatment, according to research by the NSPCC.
The charity said that some of those turned away included children whose problems stemmed from abuse.
Nearly 40,000 young people did not meet the criteria for receiving help.
The charity is concerned that the criteria for allowing children access to treatment is too strict in some areas.
The government said it was boosting funding to meet an increasing demand for these services.
The NSPCC said a time bomb of serious mental health conditions was being created and reported cases of abuse were soaring in the UK.
Figures from 35 mental health trusts across England show that a total of 186,453 cases were referred by GPs and other professionals for help, but 39,652 children did not receive it.
The most common reason was that the child did not meet the clinical threshold for receiving help from Child and Adolescent Mental Health Services (Camhs).
Children and young people and their families can be referred to Camhs if children are finding it hard to cope with family life, school or the wider world. More