Objective: As the number of mindfulness-based interventions (MBIs) for children has been constantly growing, there is a benefit to be derived for a comprehensive study that gathers what mindfulness activities have been reported to be effective at particular developmental stages, and how these practices have been modified to make them age-appropriate. Method: To address this problem, the content of 26 (cluster-) randomised controlled studies was analysed to identify any quantitative and qualitative differences. Results: The duration of MBIs varied greatly ranging from 4 to 25 weeks, 8 to 144 sessions, and 3 to 45 h, with session duration from 3 to 90 min, which is an important factor to take into account. However, there were no age group differences due to number of sessions or program length. MBIs for early childhood were more likely to use mindful movement practice, psychoeducation, and story-based context. In case of other components, such as breathing awareness or working with thoughts and emotions, qualitative differences were found between early and middle childhood regarding how the activities were implemented age appropriately. Conclusions: The present study offers practical implications and distinguishes differences in mindfulness activities for early and middle childhood. Important developmental considerations have been raised for future program developers and practitioners.