This cross-sectional survey adopting a multiple-informant perspective explores the factors that influence perceived quality (i.e., therapeutic alliance and satisfaction) in an outpatient setting within child and adolescent mental health services (CAMHS). A total of 1433 participants (parents, n = 770, and patients, n = 663) attending or having attended (drop-out) outpatient units participated in the study. The outcome measures were satisfaction (Client Satisfaction Questionnaire) and the therapeutic alliance (Helping Alliance Questionnaire). The determinants of these quality indicators were socio-demographic variables (e.g., age, gender, and mother's socio-economic status), factors related to the extent of difficulties (number of reasons for the consultation, number of people who referred the child to the CAMHS), the approach to treatment at outset (agreeing to the consultation, feeling reassured at the first appointment), the organizational friendliness (secretary, waiting room, waiting time for the first appointment) and the organization of the therapy (frequency of sessions, time for questions, change of therapist). The approach to treatment at outset, accessibility by phone, satisfaction with the frequency of the sessions and having enough time for questions were the factors that consistently explain the quality indicators from both perspectives (patients and parents). In contrast, the socio-demographic variables as well as the extent of difficulties and factors related to the organizational friendliness and the organization of the therapy (frequency of sessions, change of therapist) were not related to the quality indicators. This study identifies key determinants of the quality indicators from the perspective of patients and parents that should be considered to improve CAMHS care quality. First appointments should be carefully prepared, and clinicians should centre care on the needs and expectations of patients and parents.