No treatment will be administered to participants in this arm until after the 4-month follow-up in-lab assessment is completed.

Treatment (6-week group Emotion Focused Family Therapy) will be administered to participants in this arm.

The Emotion Focused Family Therapy (EFFT) intervention for this study will be delivered to groups of four to ten parents over six weekly sessions of two hours each. The goal of EFFT is to teach and empower parents to coach their child to process the emotions at the source of their mental health symptoms, to increase their child's adaptive behaviours, and to repair emotional injuries within the parent-child relationship. EFFT also targets parents' own emotional challenges that may prevent them from feeling capable or being able to support their child's treatment.

Each EFFT session will include introductions, psychoeducation about emotion and psychopathology, introduction of skills, experiential activities (e.g., chair work), open discussion and feedback, and homework activities covering the foci of EFFT: emotion coaching, behaviour coaching, therapeutic apology, and parental emotion blocks.

This outcome will be measured through the Kessler Psychological Distress Scale (K10), a 10-item self report measure of psychological symptoms that parents will complete. Parents will rate the extent to which they are affected by various psychopathology symptoms on a five-point Likert scale from 1 (None of the time) to 5 (All of the time), with greater total scores (range = 10-50) indicating more severe psychopathology.

This outcome will be measured through the Behavior and Feelings Survey (BFS), a 12-item measure of child psychopathology symptoms to be completed by both parents (parent-report) and children (youth-report). Parents and children will rate items on a five-point Likert scale from 0 (Not a problem) to 4 (A very big problem), with greater scores (Internalizing subscale range = 0-24; Externalizing subscale range = 0-24; Total score range = 0-48) indicating more severe psychopathology.

This outcome will be measured through the Difficulties in Emotion Regulation Scale (DERS), a 36-item self-report measure of emotion regulation difficulties to be completed by parents. Parents will indicate the frequency with which they experience difficulties with emotion regulation on a five-point Likert scale from 1 (Almost never) to 5 (Almost always), with greater total scores (range = 36-180) indicating greater difficulties with emotion regulation.

This outcome will be measured through the Negative Emotionality subscale of the Child and Adolescent Dispositions Scale (CADS), a scale of child emotion regulation to be completed by parents (parent-report) and children (youth-report). Parents and children will rate the seven Negative Emotionality items on a four-point Likert scale from 1 (Not at all) to 4 (Very much/very often) with greater scores (range = 7-28) indicating poorer emotion regulation.

This outcome will be measured through the Coping with Children's Negative Emotions Scale (CCNES), a 12-item self-report measure of parental emotion parenting practices in response to children's expression of negative emotion. Responses are divided into six subscales representing six distinct types of parental responses. The responses will be grouped in two overall scores: Supportive (Expressive Encouragement, Problem-Focused, and Emotion-Focused Reactions) and Unsupportive (Punitive, Minimizing, and Distress Reactions) practices. Parents will rate the likelihood that they would respond in these distinct ways on a seven-point Likert scale from 1 (Very unlikely) to 7 (Very likely), with greater overall scores (range = 1-7) indicating greater levels of each type of parental response.