As mobile technologies become more ubiquitous, design work at the intersection of mixed reality and embodied learning is growing. While much of this work focuses on designing technologies and environments for children, we contribute a unique perspective of children as designers of these technologies. In this paper, we explore how children embody and debug computational algorithms through designing their own mixed reality games. We conducted two afterschool workshops with 19 middle school aged children (3 girls, 16 boys, ages 10-13) during which participants designed mobile, location-based games with mixed reality technologies about local plants and animals. Findings reveal how participants across workshops embody a key game mechanic (digitally spawning characters in the real world) by engaging in an iterative digital-to-physical-to-digital debugging process that led to their understanding of the underlying computational algorithm. We further present design considerations for authoring platforms that allow children to design with mixed reality, place-based technologies.