This study aims to design an authoring system of robots and IoT-based toys for creating a scenario-based interactive learning environment for young English as a Foreign Language learners. This study adopts a design-based research approach to investigate the pedagogical needs, the critical features and usability of such a authoring system. Twelve adult participants, including EFL teachers, parents, non-technical, and technical professionals, voluntarily participated in this study to co-develop and evaluated the authoring system through iterations of needs collection, system design, system testing and refinements. An authoring system, consisting of IoT-based toys, a robot and a script editor, was finally developed taking four cycles of iterative improvements in 18 weeks. Eleven pedagogical needs concerning children's language and cognitive development, better children-robot interaction and practical issues were identified. The modifications made across different versions of the authoring system were presented in detail. The results also show that the issues of effectiveness, robustness and efficiency were the most frequently identified, and that the system had achieved good usability after four rounds of design and refinements. Design principles and guidelines also merged as a result providing insights into the effective design of Robot Assisted Language Learning in particular and Technology-Enhanced Language Learning in general.