Background: Designing and implementing a truly self-determined physical activity (PA) intervention has required excessive amounts of labor and expenses that, until recently, have made it prohibitively costly to implement in the field at scale. Methods: Guided by self-determination theory, and harnessing the power of consumer-grade interactive technologies, we developed the Virtual Fitness Buddy (VFB) Ecosystem. Designed to foster intrinsic motivation toward adopting PA as a lifestyle change in 6-10-year-old children, the Ecosystem features a mixed-reality kiosk which houses a personalized virtual pet for each user. Each time a child visits the kiosk, the pet (a mid-sized dog) automatically detects its owner based on the data from a child's Fitbit, assists the child in setting daily PA goals and provides tailored feedback on the child's PA progress. The pet alerts parents in real-time by sending text messages and relaying the parents' response to the child, so that parents and children can remain connected about the child's PA progress even when they are physically apart. We aim to implement the kiosk in 12 after-school sites, plus use 12 additional sites as controls, where children can still set and view progress toward their PA goals without access to a virtual pet. Conclusion: The VFB Ecosystem represents a new generation of technology-mediated health interventions for children to promote sustainable PA lifestyle changes. Because the VFB Ecosystem is a cost- and labor-effective solution that integrates consumer-grade technology with low barriers for continued use, it has the potential for rapid diffusion and widespread public health impact.