The therapeutic relationship is organized triadically between children, therapists and parents, yet little research focuses on its evolution as a change mechanism integrating these perspectives. Develop a comprehensive model of the therapeutic relationship as a change mechanism in different phases of psychotherapy, from the perspective of children, parents and therapists. A grounded theory analysis integrated findings from a retrospective and a qualitative longitudinal study of the therapeutic relationship with children and parents as change factors based on semi-structured interviews and children’s drawings. Initially, therapists’ close, flexible attitude improved children’s and parents’ motivation for psychotherapy. Later, a child-centred, playful relationship fostered children’s affection and trust in the therapists, promoting emotional and socio-affective changes. Lastly, as a greater bond developed, children internalized the therapist as a significant figure, and deeper changes emerged. Parents’ changes were eased by further trust and collaboration in the relationship with the therapist, and changes in the child-parent relationship emerged. Constructing positive therapeutic relationships requires participants’ evolving dispositions. These relationships shape progressive changes in children and their parents.