The Assisting Hand Assessment (AHA) provides a new perspective of hand function evaluation relevant for children with unilateral upper limb disabilities. It measures how effectively the involved hand is actually used for bimanual activity, which, for these children, might be the most important aspect of their hand function. The aim of this paper is to report the conceptual framework and the evidence for validity, reliability, and responsiveness to change for the measures. Previously, the AHA has been evaluated for children aged 18 months to 5 years and excellent inter- and intrarater reliability was demonstrated. This paper reports further evidence of construct validity and reliability for the AHA measures involving an extended age range of children with hemiplegic cerebral palsy or obstetric brachial plexus palsy from 18 months to 12 years of age (mean age 4y 11mo [SD 2y 9mo] range 18mo-12y 8mo). A Rasch measurement model was used to analyze 409 assessments from 303 children (170 males, 133 females). The analysis generated a scale demonstrating large capacity to reliably separate and spread personal ability measures, indicating sensitivity to change and a hierarchy of the items ranging them from easy to hard. Aspects of item fit, relationship between age and ability measures, and development of assisting hand function are discussed.