Many population data sources do not routinely collect data of children under 12, despite research showing that mental health, self-injurious behaviour, and substance ingestion can have severe consequences in this age group. This article used 6 years (January 2012 to December 2017) of ambulance attendance data from the Australian state of Victoria to characterise mental health, self-injurious behaviour, and substance ingestion in children aged 7–11. The authors compared this group to older children aged 12–17. They found that in comparison to those aged 12–17 (n = 26,778), a smaller number of children aged 7–11 years (n = 1558) were experiencing serious harms, with mental health symptomology the most common harmful outcome. Self-injurious behaviour significantly increased in both age groups throughout the study period. For mental health, self-injurious behaviour and substance ingestion in the 7–11 age group, males were significantly over-represented. These aged 7–11 were more likely to ingest pharmaceuticals, rather than alcohol or illicit substances, and suicidal ideation was the most common self-injurious behaviour in this age group. Findings suggest that data collection needs to occur specifically in the 7–11 age group, and importantly, services and interventions to improve mental health and wellbeing will need to be specifically designed and targeted at this age group.