To measure parents’ representations of their child’s mental health problems, the validity of the Parents’ Illness Perception Questionnaire–Children’s Mental Health (PIPQ-CMH), a modified version of The Illness Perception Questionnaire–Revised, was established to explore the link between parents’ representations and (a) child problem severity, (b) parental adjustment, and (c) treatment acceptability. Parents (N = 487) of 4- to 15-year-old children (68% boys) from five children’s mental health centers across Southwestern Ontario, Canada completed the PIPQ-CMH along with additional measures to assess validity. The Brief Child and Family Phone Interview was used to assess problem severity, the Depression and Anxiety Stress Scale was used to assess parental adjustment, and an adapted version of the Treatment Acceptability Questionnaire was used to assess the acceptability of five types of psychological treatment. Confirmatory factor analysis demonstrated that the PIPQ-CMH was a reasonable fit to the data. Cronbach’s alpha and test–retest reliabilities were above .70. Significant relationships were found between parents’ perceptions (e.g., timeline, controllability, consequences, illness coherence, emotional representations) and child problem severity, parental adjustment, and treatment acceptability. Preliminary construct validity exists for the PIPQ-CMH as a measure of parents’ representations of child mental health problems. This measure should help to inform the impact of parent representations on the treatment process.