In recent years, the concept of "deeper learning" has generated great interest in the field of preschool education in China. However, deep learning in the preschool stage is different from that in school education, as it requires explaining content characteristics, developing observational tools to identify deep learning, and exploring ways to guide children's deep learning through play. This topic is also relevant worldwide, as there is a growing interest in the relationship between play and young children's academic learning. The aim of this study is to address these issues by examining the content of deeper learning based on the ideal state of symbolic games -- "mature play" in the preschool stage. The study adapted the structure of observed learning outcomes (SOLO) taxonomy, a renowned observational tool for developing students' deep understanding, for use in preschools and investigated teachers' guidance for preschool children during symbolic games. A comparative action study spanning two semesters and involving two groups of children (free-play vs. integrated-play strategies) was conducted. Results revealed that combining free-play and guided-play strategies was essential for promoting children's deep learning. The use of the adapted SOLO taxonomy facilitated the dynamic assessment of children's learning and assisted teachers in making informed decisions about play strategies. This study provides important insights into the potential benefits of using observational tools to support teachers in promoting deeper learning in preschool children.