Inclusion Criteria:  Age 6-17 years Diagnosis of Bilateral CP: diplegia, triplegia, or quadriplegia Ability to maintain head and upper-thorax steady for 10s during supported sitting at mid-ribs or on-lower ribs: SATCo ≥ 3-4 Gross Motor Function Classification Systems-Expanded & Review (GMFCS) levels III or IV Cognitive capacity to follow basic verbal instructions (i.e., "do not put your hands on your lap" or "follow and reach the toy")  Exclusion Criteria:  Current medical illness unrelated to CP Severe dyskinesia that prevents the child from maintaining sitting while executing volitional movements without losing balance History of recurrent seizures (daily) or drug-resistance epilepsy Severe Spinal Deformities: scoliosis >40◦ and/or kyphosis >45◦ Spinal Arthrodesis High level of trunk control during sitting in case of GMFCS III (SATCo = 6 or 7) Severe spasticity in both upper extremities (Modified Ashworth Scale = 4) Local chemodenervation therapy in spastic muscles (e.g., botulinum toxin or phenol injections) in upper or lower extremities in the previous 3 months or planned during the study Current treatment with intrathecal baclofen Orthopedic surgery in affected arm or spine in last 12 months (if medically contraindicated) Major surgery in the previous 6 months prior to the study (if medically contraindicated)