To explore the impact of medical complexity, defined by the number of chronic conditions, on the complexity of care, as described by the frequency of nursing diagnoses (NDs) and nursing actions (NAs), in paediatric patients. Retrospective observational study. This study was conducted in an Italian university hospital and involved the analysis of electronic health records for neonatal and paediatric patients who were consecutively admitted from January to December 2022. The sample was classified into three categories-non-chronic, single chronic and multimorbid patients-according to their clinical profiles. NDs recorded within the first 24 h from patient hospital admission and NAs performed throughout the hospital stay were counted for each group. Distinct variations in the prevalence and patterns of NDs and NAs were observed across different levels of medical complexity. A significant moderate positive correlation between the number of NDs and NAs was found. However, the frequency of NDs did not directly correlate with the number of chronic conditions. Conversely, a weak but significant negative correlation was identified between the quantity of NAs and the number of chronic conditions. While the frequency of NDs showed a stable but decreasing trend as the number of chronic conditions increased, a higher number of chronic conditions were associated with a lower quantity of NAs. We discovered a notable variation in the complexity of care across varying levels of medical complexity in paediatric patients. Our findings suggest that the complexity of care does not necessarily correspond to the degree of medical complexity. The observed negative relationship between the number of chronic conditions and the quantity of NAs underscores the need for further research to explore this unexpected finding and its implications for clinical practice. Without the adoption of standardised nursing terminologies, such as nursing diagnoses (NDs) and nursing actions (NAs), assessing the complexity of care in paediatric settings can be challenging. Integrating clinical nursing information systems that incorporate standardised NDs and NAs into electronic health records is crucial for accurately documenting and analysing the complexity of care and its relationship with medical complexity. In paediatric patients, the frequency of nursing diagnoses (NDs) at hospital admission is significantly associated with the quantity of nursing actions (NAs) delivered during hospitalisation. However, there is no correlation between the frequency of NDs and medical complexity, as defined by the number of chronic disorders. Specifically, the frequency of NDs decreases with increasing medical complexity, while the quantity of NAs is negatively associated with the number of chronic disorders. This indicates that the complexity of care cannot be inferred solely from medical complexity, and additional factors need to be explored. These findings enhance understanding of how complexity of care relates to medical complexity in paediatric patients. Insights into the prevalence and patterns of NDs and NAs can benefit nurses, managers, researchers and policymakers by informing clinical and organisational decision-making to ensure high-quality care. The study adhered to the RECORD Statement. Patients, service users, caregivers or public members were not directly involved in the design, conduct, analysis and interpretation of data or in writing this paper. Patients contributed only to data collection.