The theory of posttraumatic growth (PTG) proposes that from life difficulties positive changes can happen, such as deepened personal relationships and an awareness of new possibilities in life. PTG can occur naturally or can be facilitated. This study aimed to promote PTG through a school-based intervention of eight sessions of 45 min each with mindfulness and character strengths practices (the so-called “The exploration of happiness during the COVID-19 pandemic”). The study conducted assessments at baseline, post-intervention, and follow-up (i.e., one month after the intervention). The post-intervention results showed that the participants in the intervention group experienced an improvement in PTG, well-being, mindfulness, strengths use, and PTS symptoms compared to the children in the control group. Furthermore, these positive changes were sustained at follow-up. The findings of this study highlight that mindfulness-based strengths practices can increase positive outcomes (i.e., well-being, posttraumatic growth) and reduce negative psychological symptoms (PTS) among children. The implications for theory and practice are discussed, and detailed appendices for practitioners are provided.