As an important construct in the cognitive process, comprehension monitoring has received much scholarly attention. Researchers have recognized comprehension monitoring as an ability closely linked with children's reading comprehension ability and working memory capacity. Evidence is also abundant to prove that comprehension monitoring skill develops with age. It remains unclear, however, how these factors interact during reading, particularly in low-grade children. Many previous empirical studies have only employed online or offline measurements to examine children's monitoring performance, which might lead to unsolid conclusions. In this study, we utilized both online eye-tracking measures and offline probe interviews to quantify the developmental features (i.e., evaluation and regulation) of comprehension monitoring skills among Chinese beginning readers. The results indicated that the comprehension monitoring performance, as quantified by eye-tracking measures, was positively related to their reading comprehension ability and working memory capacity. Moreover, the first-graders' performances lacked online regulation skills during the error-detecting tasks, while second-graders had relatively developed online monitoring performance. Additionally, the eye-tracking measures were found as a predictor for children's performances in probe interviews, as the readers with high comprehension ability and working memory capacity successfully reported more errors embedded in the self-designed reading materials. Therefore, the findings support the claim that children's comprehension monitoring is a developing skill associated with reading comprehension and working memory capacity and further question the existence of comprehension monitoring skills in beginning readers, especially first-graders.