“68. What does the concept of the best interests of the child in article 3.1 encompass? ... the Committee … [suggests] that the concept [has] three dimensions: (a) a substantive right of the child to have his or her best interest assessed as a primary consideration when different interests are being considered in order to reach a decision on the issue at stake; (b) an interpretive principle, irrelevant to the present appeals; and importantly (c) a rule of procedure that, whenever a decision is to be made that will affect an identified group of children, the decision-making process must include an evaluation of the possible impact of the decision on them.”