Cognitive fatigue is an insufficiently recognized dimension in the assessment of children and adolescents with mental and neurodevelopmental disorders. In clinical practice, it is rarely reported directly as fatigue; more often, it appears indirectly through a decline in work pace, increasing errors, task disorganization, irritability, withdrawal, refusal to cooperate or rapid discontinuation of activity. Such behaviours may be misinterpreted as lack of motivation, resistance to demands or laziness. The aim of this paper is to present cognitive fatigue as an indicator of reduced tolerance of mental effort, overload of executive control systems and increased regulatory cost of task performance. This is a narrative and clinically oriented review; it is not a systematic review or meta-analysis. The paper proposes a framework based on three dimensions: competence, capacity and recovery. Competence refers to understanding a task or possessing a skill; capacity refers to the ability to use this competence over time and under cognitive load; recovery refers to returning to equilibrium after mental effort. The role of sustained attention, working memory, inhibitory control, processing speed, cognitive flexibility, sleep, stress and sensory load is discussed. Particular attention is paid to the distinction between task completion and the cost of task performance, which may become visible only after the activity has ended. Cognitive fatigue is not treated as a separate diagnostic entity, but as a transdiagnostic functional dimension. Including it in neuropsychological assessment may improve the interpretation of child behaviour and support more adequate clinical, educational and therapeutic interventions.