Dog-assisted therapy (DAT) has shown benefits in people with mental health disorders. A child psychiatric day hospital would be a suitable setting to implement DAT and evaluate the benefits in a pediatric population. Methods: Mixed methods research in a naturalistic setting was considered in this pre-post quantitative study including 23 children under 13 treated in a day hospital over 2 years. Quantitative analysis included the number of emotional and behavioral outbursts and attendance rate and self-control and social impairment questionnaires completed by family members and therapists. In the qualitative study, the experiences of 12 mental health professionals involved in DAT were documented through semi-structured interviews. Results: On DAT days, there were fewer emotional and behavioral outbursts and higher attendance. Significant differences were obtained between pre- and post-test scores on the SCRS and the SRS-2 completed by the therapists, while no significant differences were obtained on the questionnaires completed by the parents. Observations based on the qualitative study were as follows: (1) DAT improves emotional self-regulation; (2) DAT could facilitate the work of therapists in day hospitals; (3) health professionals displayed uncertainty due to a lack of familiarity with DAT. Conclusions: DAT improved emotional self-regulation, attendance rate and self-control and social response in children with mental disorders attending a day hospital.