Artificial companion agents have the potential to combine novel means for effective health communication with young patients support and entertainment. However, the theory and practice of longterm child-robot interaction is currently an under-developed area of research. This paper introduces an approach that integrates multiple functional aspects necessary to implement temporally extended human-robot interaction in the setting of a paediatric ward. We present our methodology for the implementation of a companion robot which will be used to support young patients in hospital as they learn to manage a lifelong metabolic disorder (diabetes). The robot will interact with patients over an extended period of time. The necessary functional aspects are identified and introduced, and a review of the technical challenges involved is presented.