Abstract The importance of secure human attachments in childhood for healthy psychological development is well-established, yet the well-being implications of child-dog symbiotic relationships are less understood. Children form strong emotional bonds with their pet dogs that meet the prerequisites for an attachment relationship. These bonds can be mutually reinforcing and beneficial and could indicate positive child well-being. However, not all child-dog relationships are positive and here we explore whether harmful and unsafe interactions are associated with poorer emotional and behavioural functioning. The aim of this study was to examine whether the type of child-dog behaviour (positive or negative) mediates the relationship between child-dog attachment and well-being indicators. Data from caregiver reports (N = 117) and child self-reports (N = 77) were collected through an online survey. The results revealed that positive child-dog interactions significantly mediated the relationship between high attachment scores and better child outcomes (higher scores for well-being, positive outlook, happiness, quality of life, higher social satisfaction, and lower loneliness), whereas the reverse was found for negative child-dog interactions, predicting lower attachment scores and worse child outcomes (negative outlook, increased loneliness and social dissatisfaction, lower quality of life). This study has identified important mechanisms through which pet dogs may pose both benefits and risks to children’s psychological well-being. These findings will aid the development and evaluation of interventions that promote positive and safe child-dog interactions and subsequent child and dog psychological health and welfare.