This Research Full Paper introduces a model for structuring playful, challenge-based learning activities drawing on a process of decontextualisation of a challenge to access its Computational Thinking intellectual and conceptual components and subsequent computationalisation of these components into computational artifacts that are recontextualised to render them attractive and accessible to school pupils. The case study follows a thick description approach to evaluating the engagement potential of the instructional design, as well as the didactic choices made during implementation. We conclude that Bebras cards and Dash robots provide considerable support for a playful engagement with computational concepts and engaging children with different scientific backgrounds in Computational Thinking. In particular, flexibility in how they are used and easy adaptation of challenge level make them useful in contexts with broad participation. Additionally we find that using robots to provide a link between the theoretical presentation of CT in the Bebras card and a physical representation and programming challenge is engaging and helps participants to focus on algorithmic concepts.