Software avatars are increasingly applied to support children's collaborative storytelling because avatars may serve as a transparent shield or as embodied agents to facilitate creativity in children. However, when using avatars, it is not easy for children to practise and refine their speaking skills by narrating their stories to an audience. The programming difficulties involved in that process tend to distract attention away from the collaborative storytelling activity itself. This study therefore proposes an interaction model featuring tangible story avatars (TSAs) that can serve as a platform to scaffold collaborative story creation, revision and narrative. A 9-week study with 16 children aged 8 years was conducted to evaluate the interaction model with the TSAs. The results suggest that the TSAs may play three primary roles in facilitating collaborative storytelling: as a tool for developing confidence, as a tool for reflective story development and as a group artefact. Researchers and educators may find it valuable to consider these three roles in designing the TSAs in support of collaborative storytelling in similar contexts.