Aim: To describe the development of social function in children with cerebral palsy (CP) classified in all levels of the Communication Function Classification System (CFCS). Method: This prospective, longitudinal population-based cohort study recruited children with CP born in Queensland, Australia. Social functioning was measured using the Pediatric Evaluation of Disability Inventory (PEDI) social function domain at 2 years, 2 years 6 months, 3 years, 4 years, and 5 years, and the PEDI Computer Adaptive Test (PEDI-CAT) social/cognitive domain at 8 to 12 years. Results: Seventy-four children provided 356 observations. PEDI-CAT social/cognitive scaled scores at 8 to 12 years were (mean [SD] n) CFCS level I, 68.6 (2.7) 45; CFCS level II, 64.0 (3.4) 10; CFCS level III, 63.5 (3.7) 4; CFCS level IV, 56.8 (5.0) 9; CFCS level V, 47.2 (5.8) 6. Scores within expected range for age (not less than 2 SD below mean) at 8 to 12 years were achieved by 35 (78%) children in CFCS level I and four (14%) in CFCS levels II to V. Forty-nine per cent of children scored at least two standard deviations below the population mean on a proxy measure of fluid intelligence. Intellectual impairment was associated with lower PEDI-CAT social/cognitive scaled scores in univariable analysis (β = −8.3, 95% confidence interval − 10.91 to −5.63; p < 0.001) but had a smaller effect when modelled together with CFCS. Interpretation: Social function attained by 8 to 12 years of age was strongly related to level of communication function (CFCS). The small number of children classified in CFCS levels II to V necessitates caution when viewing these individual CFCS level trajectories. What this paper adds: There is a strong relation between social functioning and Communication Function Classification System (CFCS) levels. At 8 to 12 years, 35 out of 45 children in CFCS level I met social functioning age expectations. Twenty-five out of 29 in CFCS levels II to V had social functioning below that expected for age. CFCS and age were more strongly associated with development of social functioning than Gross Motor Function Classification System or Manual Ability Classification System and age.