Digital transformation and the need to foster prerequisites of engineering thinking in preschool and primary school children require an operational, rather than declarative, model of educational «seamlessness» ensuring coherence between educational levels and integration of technical, humanitarian, and artistic components. The study aimed to develop and test the pedagogical design of a seamless STEAM trajectory as a technology for structuring the educational process, providing continuity, structural consistency, and interdisciplinary integration, using the Championship «START KIT: Constructing, Instructing, Technologizing» as a case. The materials included Russian regulatory documents, scholarly literature on pedagogical design, STEAM, and seamless/continuous education, and empirical data from the Championship. Methods comprised analysis, modeling, instructional design (ADDIE), content analysis, expert evaluation, and questionnaires. The study differentiates «continuous» and «seamless» education, defines the features of seamless education, substantiates the concept of a seamless educational trajectory, identifies its types (conceptual, architectural-methodological, environmental), and characterizes a STEAM trajectory. The pedagogical design was implemented through a STEAM task architecture, unified assessment criteria, shared digital interfaces, and an open repository of construction practices, complemented by the modular extension «From Kitten to KIT». The novelty lies in clarifying the conceptual framework of pedagogy and empirically verifying the proposed types and features of seamless trajectories on the basis of the Championship.