To assess upper extremity functioning of children with unilateral transverse upper limb reduction deficiency, using standardized instruments, and to investigate their validity and reliability.Cross-sectional study.Twenty subjects aged 4-12 years; 9 prosthetic users and 11 non-users.The Assisting Hand Assessment, Unilateral Below Elbow Test, Prosthetic Upper extremity Functional Index and ABILHAND-Kids were assessed in all children. Users were tested with and without their prosthesis. We compared results of users and non-users, and of users with and without their prosthesis. Validity was determined by testing hypotheses and correlations with other measures. Test-retest reliability was assessed from repeated measurements in 10 children.Children with an upper limb reduction deficiency performed well on daily activities. They could use their prosthesis in 68% of the activities, but were currently using it in only 30%. Children find their prosthesis useful for specific activities, rather than for daily activities in general. The Assisting Hand Assessment and Prosthetic Upper extremity Functional Index showed best validity; test-retest reliability was good to excellent.The use of standardized instruments adds relevant information on functioning of children with an upper limb reduction deficiency. We found additional support for validity and reliability of, in particular, the Assisting Hand Assessment and Prosthetic Upper extremity Functional Index.