Background Self-harm and suicidality are common presentations in children and adolescents requiring a mental health inpatient admission. Although there are several studies on adolescents, there is relatively limited research into childhood self-harm and suicidality during such admissions. Methods A retrospective electronic file review was conducted on all children discharged from a national mental health inpatient children’s unit over a 6-year period. Several independent variables were compared between self-harm/suicidal and non-self-harm/non-suicidal children. Separate analyses investigated changes in self-harm/suicidality, functional outcomes, and risk assessment ratings between admission and discharge. Results A total of 105 children were included in this study. During admission, 65.7% of them reported self-harm thoughts, 61% engaged in self-harm, 50.5% expressed suicidal thoughts, and 14.3% engaged in suicidal behavior. Thoughts and acts of self-harm were associated with previous self-harm, longer admissions, and Attention Deficit Hyperactivity Disorder. Suicidality overlapped with self-harm and was strongly predicted by previous self-harm. The prevalence of self-harm and suicidal thoughts and acts significantly decreased during admission. Children in the non-self-harm group had marginally better functional outcomes upon discharge compared to those in the self-harm group. Children and parents/caregivers were similarly satisfied with treatment, irrespective of children’s self-harm/suicidality. Conclusions Self-harm and suicidality were widespread among children admitted to hospital. Better understanding of the mechanisms and factors related to self-harm and suicidality in this age group could help mitigate associated risks and improve existing safety strategies.