There is growing interest in parental psychological flexibility, understood as parents’ ability to enjoy an optimal relationship with their children by accepting their negative emotions and thoughts in the present moment, while remaining child-conscious, in value-based parenting. Parenting can sometimes be stressful, especially when faced with the varied and changing challenges of parenthood. Parental stress, in turn, has been related to high levels of psychological inflexibility. So, this study aimed to analyze the relationship of parental psychological flexibility with parental stress and its impact on children’s psychological adjustment. The sample consisted of 909 families with children aged 3–18 years who answered the following questionnaires: The Parental Acceptance Questionnaire (6-PAQ), which measures psychological flexibility, the Parental Stress Scale (PSS), and the Strenght and Difficulties Questionnaire (SDQ), which studies the general psychological state of children and adolescents by screening for emotional and behavioral difficulties. These variables were analyzed by difference analysis according to the level of psychological flexibility, regression analysis, and mediation analysis. The group of parents who scored highest on psychological inflexibility scored high on parental stress and the child psychological adjustment scales, except for the prosocial behavior subscale. The results show that parental psychological inflexibility is related to higher levels of parental stress, and in turn, has an impact on greater psychological maladjustment in children. These relationships are analyzed from their clinical and educational implications as factors and objectives to be incorporated into family intervention.