Decisional privacy offers individuals the freedom to act and to make important decisions about how they live their lives, without unjustifiable interference from other individuals or the state. Children's perceived vulnerability, incapacity for rational decision-making and dependence on adults have been used to justify depriving children of decisional privacy rights and subjecting them to the exercise of adult power over the conditions of their lives. The aim of this paper is to articulate a theory of children's decisional privacy. It is argued that decisional privacy is valued as a condition that enables individual autonomy. A relational, gradual conception of autonomy is advanced, to explain how children can be recognised as having the capacity for autonomy, and in some circumstances, actual autonomy. This paper presents four fundamental principles of a children's rights approach to decisional privacy, which collectively serve to enhance children's meaningful participation in decision-making about their best interests, consistently with children's evolving capacities and the receipt of appropriate parental direction and guidance. The theory developed in this paper presents an opportunity for adult decision-makers to reflect upon how they make decisions for and about children, and how children can play a meaningful role in those decision-making processes.