Objective <p>Mindful awareness and metacognition play protective roles in children's mental health; however, their developmental trajectories and interconnections remain unclear. Drawing upon Monitor and Acceptance Theory (MAT), this study investigated whether mindful awareness and metacognitive abilities increase with age and how their development influences one another, hypothesizing that growth in one may promote growth in the other.</p> Method <p>A 3 wave longitudinal study was conducted with 620 elementary school children in Guangzhou, China. Data were collected at baseline (T1), 5&#xa0;months later (T2), and 10&#xa0;months later (T3). Covariates including grade, gender, only-child status, and "left-behind child" status were controlled for in the analyses.</p> Results <p>(a) Mindfulness awareness showed a small and non-significant increase over time, with a slower rate of growth in later stages; (b) metacognitive abilities increased over time; (c) initial mindful awareness levels and initial metacognitive ability levels were positively correlated; (d) the development speed of mindful awareness ability was positively correlated with metacognition growth; and (e) lower initial levels of metacognitive ability were associated with faster subsequent growth in mindful awareness.</p> Conclusions <p>These findings highlight the associations between mindful awareness and metacognition across middle and late childhood, offering insights for mental health interventions aligned with children’s developmental trajectories.</p> Preregistration <p>This study is not preregistered.</p>