Both empirical and clinical evidence with heterosexual parents and their biological children has shown the significant influence of early family interactions on children’s socioemotional and cognitive development during their first years. Yet, very little research has applied family-level assessment to families who are diverse with respect to parents’ gender and sexual orientation, and child’s method of conception. The present cross-sectional study compared 24 lesbian mother families with donor-conceived children and 24 heterosexual parent families with spontaneously conceived children with respect to triadic interaction quality and the family alliance (i.e., emotional and interactional coordination during family activities), as observed during the Lausanne Trilogue Play procedure. All parents were first-time parents, White, cisgender, residing in Belgium, had an upper-middle socioeconomic status, and a child aged 3–74 months ( M = 21.00, SD = 17.72). Across family types, triadic interactions demonstrated similar scores in each family alliance dimension, characterized by appropriate levels of participation, organization, focalization, and affect sharing. The results have clinical implications for the use of the LTP as both a clinical assessment and a tool to reinforce and intervene with lesbian coparents. Family psychologists may find the results particularly informative when working to support coparenting relationships among diverse families.