Designerly play has been identified as a fundamental component of childhood learning (Baynes, 1994; Petroski, 2003). However, as students enter grade one and beyond, the increasing academic focus has resulted in the loss of opportunities for designerly play (Zhao, 2012). At the same time, there are increasing calls to increase the number, skill, and diversity of STEM workers (Brophy, Portsmore, Klein, & Rogers, 2008). The robotics based Elementary Engineering Curriculum (Heffernan, 2013) - used by students in this study - and other similar projects have the potential to increase the STEM pipeline but elementary engineering is not well-understood. Research is needed to understand how to teach engineering to students as their cognitive, motor, and social skills rapidly develop in elementary school (Alimisis, 2012; Crismond & Adams, 2012; Mead, Thomas, & Weinberg, 2012; Penner, Giles, Lehrer, & Schauble, 1997; Roth, 1996; Schunn, 2009; Wagner, 1999). The literature review and theoretical frameworks chapters of this study determined the most relevant theoretical frameworks, engineering design process models, and existing research that is relevant to a cross-sectional case study of six grade 2 and six grade 6 elementary robotics students in the context of established K-6 elementary robotics curriculum (Heffernan, 2013). Students were videotaped doing an open-ended engineering task based on LEGO robotics using talk-aloud (Ericsson & Simon, 1993) and clinical interview (Ginsburg, 1997) techniques. The engineering design processes were analyzed and compared by age and gender. Significant differences were found in final projects and engineering design process. However, the differences were not, for the most part, related to development or gender, but were related to the complexity of the ride they tried to build and the skills and structural knowledge they brought to the task. The key factors identified consisted of three executive function process skills of cognitive flexibility, causal reasoning, and planning ability, three domain specific process skills of application of mathematics and science, engineering design process skills, and design principles of stability, scale, and the structural knowledge they had of LEGO robotics, most pointedly, LEGO connection knowledge. Implications of these findings for teachers are given.