Introduction Children with cerebral palsy (CP) in the form of spastic hemiplegia experience numerous difficulties concerning an affected upper limb such as reaching for objects, gripping or manipulating them. These limitations affect their everyday activity. Conducting an effective and simultaneously an interesting therapy aimed at meeting the child’s individual needs and improving upper limb function is a challenge for a physiotherapist. The aim of the study was to assess the effectiveness of upper limb therapy carried out within the project titled “The Pirate Group” based on Constraint-Induced Movement Therapy (CIMT) and Bimanual Training (BIT) conducted in a specially arranged environment. Material and methods The research included 16 children with CP in the form of spastic hemiplegia. Mean age of the study participants was 4.23 years. The children underwent a two-week Constraint-Induced Movement Therapy (CIMT) combined with Bimanual Training (BIT). In order to evaluate the effects of the therapy, each child underwent the Assisting Hand Assessment (AHA) prior to the therapy and after its completion. Results Statistical analysis revealed a significant difference (p<0.05) between the results of AHA prior to and after the therapy (t(14)=9.12, p<0.0001). An improvement in the affected upper limb function was noted in all the children participating in the research. Conclusions The project titled “The Pirate Group”, based on CIMT and BIT is an effective therapeutic intervention which improves spontaneous activity of the affected upper limb in children with hemiplegia.