Over the last two decades, a remarkable number of studies have examined the role of engineering education in supporting knowledge and skill building among children. In this paper, we present a synthesis of this literature to evaluate the added value of pre-college engineering design experiences at the elementary level, and ways researchers have gathered evidence of children's development of this knowledge and skills. We have conducted a systematic literature review. The initial search across four different databases with relevant keywords resulted in over 3000 journal articles and conference papers. In a two-rounds screening, by applying the inclusion criteria first to titles and abstracts, and then to the full text, 78 peer-reviewed publications were included in this systematic literature review. We summarized and synthesized empirical evidence of student learning and engagement in engineering design activities, and approaches they were assessed and elicited. The thematic analysis revealed five categories of knowledge and skills: (1) disciplinary content knowledge; (2) engineering design practices, (3) engineering thinking skills; (4) professional skills; and (5) career awareness. Additionally, the assessment and evaluation approaches that were used for each of these categories were identified and discussed. The findings provide collective evidence of variety in student learning opportunities but also suggest the need to carefully define these knowledge and skills. Since a wide range of evaluation approaches were used to capture similar outcomes, we conclude that it is imperative to develop a shared research agenda by carefully defining the knowledge and skills we expect children to learn.