Children and adolescents with cerebral palsy (CP) show a reduced physical activity (PA) level compared with their typically developed peers. This study aimed to identify potential predictors of objectively evaluated habitual PA using data from a national clinical registry—the Cerebral Palsy Follow-Up Program (CPFP)—and proxy-reported questionnaires. Data from the CPFP database showed that female sex was associated with a lower PA level. Additional questionnaire data revealed age, popliteal angle limitation, Pediatric Outcomes Data Collection Instrument–Sports and Physical Functioning Scale score, screen time, Functional Mobility Scale score at 50 m, and fatigue as predictors. The proposed models can be used for the prediction and early detection of the PA level and consequently for the potential improvement among ambulant/semiambulant individuals with CP. Further research should investigate the predictive impact of personal, social, and environmental factors on the PA level and the gap in PA levels between girls and boys.