Background: In this paper, we investigate the relationship between theory and design in the context of creating digital games to support children's development of scientific expertise. Synthesis: Theoretically, we consider two frameworks--Knowledge in Pieces (or KiP) and Science as Practice (or SaP). While KiP is a theory about the structure of human knowledge, SaP is a theoretical perspective about the development of scientific expertise that emphasizes the deeply intertwined nature of conceptual development and the development of epistemic and representational practices. We then synthesize research on modeling and games from our research group and others to create a composite argument and theoretical framing for the importance of "disciplinary integration" in the design of digital games for science learning. Conclusions: We show how shifting from KiP to SaP as the underlying theoretical anchor in our designs results in shifting from focusing on "conceptual integration" to focusing on "disciplinary integration." We first present our initial framing and design around conceptual integration. We then trace the evolution and rationale for disciplinary integration across our development and research on four game platforms. Finally, we discuss the implications and generalizability of disciplinary integration across the design of digital games for science learning.