Scottish Children's Services Coalition made the call after "disappointing" waiting times for appointments were revealed in the region.
Figures show in the Lothians 15% of children had to wait more than a year.
Less than half of the 665 children referred were seen within the target of 18-weeks.
Two years ago the Scottish government set a target for Mental Health services that 90% young people should be seen within 18 weeks.
The figures for treatment from Child and Adolescent Mental Health Services (CAMHS) relate to the period between October and December 2016.
A spokesperson for the Scottish Children's Services Coalition, a network of support groups,  said: "The figures from NHS Lothian indicating that more than half of those who started their treatment between October and December had been waiting more than the Scottish government target of 18 weeks is clearly disappointing.
"In addition, of the 101 children and young people in Scotland who had been waiting for more than a year before they started treatment, these almost all relate to NHS Lothian.
"We know that half of all diagnosable mental health problems start before the age of 14 and 75% by the age of 21.
"As such it is vitally important that an action plan is put in place for NHS Lothian, radically improving mental health services and increasing investment in these, with an overall aim of ensuring that children and young people get the help they need, when they need it.
"A target of 18-weeks is in itself still far too long.
"Families usually experience months of waiting even before a referral to CAMHS.
"The consequent delay in diagnosis and appropriate support can lead to a crisis situation for the child or young person concerned, as well as for their family, and the need for costly extra resources to address this.
"We hope that NHS Lothian takes account of these figures and takes urgent action to address this matter."
Alex McMahon, NHS Lothian's executive director of nursing, midwifery and allied health professionals, said: "More young people than ever before are accessing mental health services and we have seen a steady rise in the number of children and young people being referred to our services.
"Unfortunately patients are having to wait longer than we would like and we are working hard to reduce this number.
"We have  invested significantly in CAMHS over the years to support the delivery of expert services for children. We are confident they will be strengthened through integration of education, social and health services.
"Our CAMHS team is working on an agreed action plan to reduce the length of time children and young people are waiting and we have approximately 200 less children and young people waiting more than 18 weeks as a result and are confident we will see further significant reductions in waiting times over the coming months."
Support groups are calling for an action plan to "radically" improve mental health services for children and young people in the NHS Lothian area.