Background: This study examined predictors of service complexity in children with comorbid intellectual developmental disorder (IDD) and mental health concerns. We examined whether patient type, safety risk, exposure to trauma, and family dysfunction were related to service complexity. Method: The study had a cross-sectional design, wherein trained clinicians assessed 330 outpatient and inpatient children with IDD and mental health problems using an interRAI ChYMH-DD instrument. Results: Inpatients experienced a greater prevalence of abandonment by caregiver than outpatients. The patient type defined relationships between age, family dysfunction, safety risk, cumulative trauma, and service complexity. Specifically, in a multivariate model, older age, higher family dysfunction, higher safety risk, and higher cumulative trauma predicted higher service complexity in outpatients but not inpatients. Conclusions: In addition to older age, poly-victimisation, family dysfunction, and safety risk need to be considered when predicting service complexity in this population.