Computer programming for young children has grown in popularity among both educators and product developers, but still relatively little is known about what skills children are developing when they code. This study investigated N = 57 Kindergarten through second grade children's performance on a programming assessment after engaging in a 6-week curricular intervention. Children used the ScratchJr programming tool to create animated stories, collages, and games. At the end of the learning intervention, children were assessed on their knowledge of the ScratchJr language and underlying reasoning. Specifically, we explored children's errors on the assessment to determine evidence of domain-specific reasoning (e.g. mathematic, verbal, causal). Results show that while all students mastered foundational coding concepts, there were marked differences in performance and comprehension across the three grade levels. Interpretation of results suggests a developmental progression inherent in programming knowledge acquisition. Implications for computer programming education and developmental research are discussed.