Health is a devolved service and this answer relates only to England.
The Health and Social Care Information Centre (HSCIC) advises that it does not currently hold data to answer the questions asked. However, it has been capturing child and adolescent mental health services (CAMHS) data since January 2016 within the new Mental Health Services Data Set. On 20 April 2016 it published the first month (January 2016) of data from this new data source on a select number of measures it developed and quality assured to a certain level with the help of stakeholders, although these data remain experimental and could be subject to change.
As at 31 January 2016, there were 104,480 people in contact with CAMHS. HSCIC is currently unable to identify those who are already receiving treatment and those who are awaiting assessment.