This study aims to investigate whether the treatment effects, in terms of goal attainment, transfer effects and impact on executive functions, of an intervention in children with cerebral palsy or spina bifida using the Cognitive Orientation to daily Occupational Performance (CO-OP) Approach are maintained over time, from immediately after the intervention to three months afterwards. A three-month follow-up study, from an intervention using CO-OP. Thirty-four children (7-16 years) each identified four goals (one untrained to examine transfer) and participated in an eleven-session intervention. Assessments were performed at baseline, immediately after the intervention and at a three-month follow-up using the Canadian Occupational Performance Measure and the Performance Quality Rating Scale. Executive function and self-rated competence were assessed at the same timepoints. Statistically significant and clinically relevant improvements in goal achievement were demonstrated for both trained and untrained goals after the intervention and were maintained at follow-up. The clinically relevant improvement in untrained goals continued to increase until follow-up. Self-rated competence increased after the intervention and was maintained at follow-up. The CO-OP intervention was effective in achieving and maintaining the children's own goals over time. The transfer effect was confirmed by higher goal attainment for the untrained goals.