The motor arousal of children with emotional and behavioral difficulties influences and is influenced by the quality of the non-verbal interactions with their parents. The aim of the present study was to validate the Parent–Child Movement Scale (PCMS), a reliable observation tool for joint dances between parents and their preschool children. To test the tool's discriminant validity scores of four mother–child groups on the PCMS: BH (both high), BL (both low), MH–CL (mother high–child low) and ML–CH (mother-low, child-high) were compared. Sixty mothers of children with dysregulation and behavior problems were filmed as they engaged in a “mirror game” in which each led the other in spontaneous movements. The mothers completed the Child Behavior Checklist (CBCL) and the Brief Symptom Inventory (BSI) questionnaires. Analysis of the distribution of the participants’ mean scores and a Kruskal–Wallis non-parametric test showed that there were significant differences between groups. Mother–child dyads in which the child scored low on the PCMS was associated with more introversion/depression problems than in dyads in which the child and mother scored high on the PCMS. A high score for the mother and a low score for the child on the PCMS was associated with more anxiety for mothers compared to mothers in groups where the mother and child had the same score. The discussion focuses on the contribution of nonverbal parameters to the assessment of depression.