Digital fabrication laboratories (Fab Labs) are accessible to the public, including children and families. However, a variety of technical skills, tools, and expertise are needed in these spaces, and Fab Lab staff--instructors working there--are in a key role in helping Fab Lab users. There is, however, a lack of research on how these instructors can inspire children's genuine participation in Fab Labs. We analyze the challenges faced by four instructors during two months of Fab Lab workshops with 7-12-year-old children. The children worked in child-only or child-parent groups on informal digital design and fabrication activities. Based on genuine participation principles derived from the literature, we designed our workshops, which included the creation of instructor guideline. At the end, we developed a post-workshop framework in which the instructors provided feedback about their performance and experiences. We provide recommendations to help instructors support children's genuine participation in digital design and fabrication.