Insights on students' own authentic design practices--nascent design practices without much adult guidance--are crucial to informing responsive facilitation of engineering design tasks. This study unpacks how elementary students interpret teacher given information about a design task, and interact with each other and given resources, to traverse a design trajectory with limited adult intervention. We present a qualitative case study of the trajectory of a 3rd grade student team working to design a solution in response to an engineering challenge, with minimum adult intervention. The design space lens, which emerges from studies of nonlinear, fluid nature of professionals' design trajectories, guides our analysis of elementary students' design trajectory. The use of design space reveals characteristics of student navigation as dictated by their interpretation of given information, and by their observation and analysis of how their solution behaves and performs. The findings show students' proactive and autonomous selection of design practices driven by the goal of solving the problem while navigating criteria and constraints. The study highlight a sophisticated trajectory that elementary students can traverse when given the autonomy. Awareness of these nascent strengths has implications for helping educators to responsively plan and facilitate design tasks.