The best interests of the child are examined as an interpretive principle and its scope in the process. For this purpose, the canon proposed by General Comment Nº 14 of the UN Committee on the Rights of the Child is used. On that document some clarifications are made that start by clarifying what it means that the principle is assimilated to a "rule of procedure". Then, the points that constitute the canon indicated are clarified based on some procedural problems that occur in family justice, referred to the right of defense, the right to appeal, the optimal assessment by the authority of the facts that affect children and the jurisdiction of the courts. The novelty offered by this article is the study of the general principle from a procedural prism, given that the doctrine mostly treats it from a substantive point of view.