Content validity describes the extent to which a measure represents, and is relevant to, the construct it aims to assess. The International Classification of Functioning, Disability and Health and derived Core/Code Sets (Sets) for autism, attention deficit-hyperactivity disorder, cerebral palsy and early developmental delay and disability are adequate to establish the content validity of measures aiming to assess functioning in young children with neurodevelopmental conditions (NDCs). This article aimed to assess the content validity of comprehensive assessments of functioning for young children with NDCs against these standards. Twenty-two common measures of functioning were evaluated for content validity against the International Classification of Functioning, Disability and Health at a domain level, with 10 measures analysed at the item-level and compared to the Sets relevant to young children with NDCs. Measures covered between 21% and 57% of the combined Set codes and 19% to 63% of codes from specific Sets. Much of this variation was between measures, with some variation due to differences between individual Sets. The percentages reflect that measures heavily focus on activities and participation areas, with environmental factors rarely assessed. These findings are useful for clinicians, policymakers, and researchers in identifying the most appropriate measures for assessing functioning in young children with neurodevelopmental conditions.