Forced displacement is a major driver of mental disorders among refugees worldwide. Poor mental health of adult refugees, particularly mothers, is also considered a risk factor for the psychological well-being and development of their children. In this study, we experimentally examine the extent to which a multifaceted psychosocial program improves the mental well-being of refugee mothers, and facilitates growth and development among children under the age of two. In partnership with BRAC, we ran a cluster randomized controlled trial on 3,500 Rohingya mother-child dyads in refugee camps in Bangladesh. Participants were given weekly psychosocial support for a year that includes psychoeducation and parenting support for mothers and play activities for both mothers and children. The intervention was largely successful and led to: (i) reductions in the psychological trauma and depression severity of mothers and children, (ii) improvements in communication, gross-motor, problem-solving, and social skills of children, and (iii) reductions in stunting, underweight, and wasting among children in the treatment group. The intervention also caused the mental health of children to be more aligned with the mental health of their mothers, implying policies targeting the mental well-being of displaced mothers can be an important stepping stone to developing psychological resilience among their children, which can help them grow into well-rounded, healthy adults.