Ninety-five children were admitted to adult mental health centres last year, according to the Mental Health Commission.
In its annual report for 2015, the Commission said the situation is "unacceptable" and needs to change as a matter of urgency.
It has also described as "odd" that the HSE cannot provide it with details on the exact number of 24-hour community residences.
Under the Mental Health Commission’s code of practice, no child is to be admitted to an adult unit except in exceptional circumstances.
However, last year of the 501 children admitted to mental health services, 95 of these were sent to an adult service.
Last year, 61 approved centres were inspected, only six of which were fully compliant with requirements, but the Commission also has remit to inspect other centres' 24-hour community services.
But the HSE has failed to provide them with information on the exact number of these, and furthermore, as the law stands the Commission does not have power of enforcement over these centres.
The Mental Health commission hopes that will change later this year.