The goal of this study was to determine whether increasing motor variability by applying varied perturbation forces to the pelvis during sitting astride would facilitate motor learning of improved trunk postural control in children with cerebral palsy (CP). Fourteen children with spastic CP were tested in two conditions: CONSTANT and VARIED force perturbations in two training sessions with each session lasted for 15 minutes. A custom robotic system was used to deliver repetitive perturbation forces to the pelvis while participants sat astride. Postural reactions to the unpredicted perturbations in the anterior and posterior directions were tested before and after each training session. Kinematics of head, trunk, and pelvis movement, and muscle electromyography signals were recorded. One session of pelvis perturbation training with either the CONSTANT (P = 0.028) or VARIED force magnitudes (P = 0.009) induced an earlier onset of trunk extensor under unpredicted perturbations. In addition, one session of pelvis perturbation training with VARIED force magnitudes induced a significant decrease in peak angle of head extension (P = 0.016), and an improvement in recruitment order of the neck and trunk muscles under unpredicted perturbations (P = 0.020). These results suggest that repeated pelvis perturbations during sitting astride may induce improvement in muscle activation onset under unpredicted perturbations in children with CP. Moreover, repeated pelvis perturbations with varied force magnitudes, which may increase motor variability, may lead to improvements in head stability and muscle activation sequence of trunk and neck muscles in response to unpredicted perturbations in children with CP.