Promotion of children's mental health (MH) plays an important role in preventing further MH problems. In Finland, the wellbeing of children is supported in early childhood education and care (ECEC), basic education, maternity clinics, and child health clinics. Providing these services for children and families requires multiprofessional collaboration. The aim of this study was to investigate the current practice of children's MH promotion, more specifically children's MH services and multiprofessional collaboration between social and health care services and ECEC/ basic education. The study focused on MH questions regarding children in 18 municipalities of Finland from the approach of socio-ecological model of health promotion. The participants of this cross-sectional study were ECEC professionals in municipal day care, basic education professionals, and primary health care professionals who work with children younger than 14. The data collected via questionnaires were analyzed using descriptive statistics, chi-squared analysis, principal component analysis (PCA), the Brown-Forsythe robust test, and Tukey HSD post hoc comparisons. A total of 482 professionals participated in this study. Overall, respondents indicated relatively poor satisfaction toward most aspects of current MH promotion. Professionals from municipalities that provided primary health care and social services independently evaluated MH promotion most positively. However, deficiencies in multiprofessional collaboration between organizations were reported. The work experience influenced professionals' perceptions of the challenges of collaboration; respondents with the least work experience were most critically concerned. These results imply that the promotion of children's MH and MH services needs to be further developed.