But if it does happen, an independent advocate should be appointed for that person, says the NHS guidelines.
This follows criticism from watchdogs and a BBC Wales investigation in which experts said the service was in crisis.
Health Minister Mark Drakeford said children "should be accommodated in the most suitable environment".
In a joint report last year, the Wales Audit Office and Healthcare Inspectorate Wales said that despite some improvement "children and young people continue to be put at risk due to inappropriate admissions to adult mental health wards".
Ministers say they are now consulting on draft guidance setting out the procedures for local health boards.
The document acknowledges that Child and Adolescent Mental Health Services (CAMHS) "have limited capacity to provide comprehensive out of hours services".
However, it says only in "very exceptional circumstances" should children be admitted to adult services.
Mr Drakeford said: "This draft guidance sets out our expectation that the admission of a young person to any inpatient unit should be a rare event.
"I expect health boards to ensure services for children and young people are fit for purpose, that care is integrated across services and that inpatient services consider the individual needs of children and young people and develop services to cater for these needs."
Children and young people being admitted to adult mental health wards should be a "rare event", says new draft guidance.