A review had been promised by Malcolm Chisholm, Health Minister, during the debate on the Mental Health (Scotland) Bill.
Children in Scotland wants the review to take account of "the need for those children and young people who require hospital treatment to be admitted to an environment appropriate to their age".
The agency says that 72 per cent of those aged under 18 are admitted to adult psychiatric wards.
Eddie Pollan, Children in Scotland's policy officer, welcomed the review and said: "We know that child and adolescent mental health services in Scotland are currently under severe pressure and that the level of mental ill health among our children and young people is increasing."
The organisation wants the Bill to place health boards under an obligation to make proper hospital provision for children with mental health problems.
Two to three pupils could be affected in every class at any one time.