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Computed Tomography (CT) provides detailed medical images but exposes patients to relatively high radiation doses, making dose optimisation especially important for children because they are more sensitive to radiation. This study evaluated the effects of CT scan parameters on organ absorbed dose in a 5-year-old paediatric computational phantom using the Geant4 Monte Carlo simulation toolkit. A paediatric MIRD phantom and polyenergetic X-ray spectra (80, 100, and 120 kVp) generated with SpekPy were used to simulate CT imaging. The investigated parameters included tube voltage, tube current, beam width, scan length, and bow-tie filtration. Results showed that increasing tube voltage and tube current increased organ absorbed dose, while the lungs received the highest dose. The high bow-tie filter reduced organ doses by 44–57%, and increasing beam width slightly increased thyroid dose. Most statistical uncertainties were below 5%. These findings demonstrate that lower tube voltage, lower tube current, narrower beam width, shorter scan length, and bow-tie filtration can effectively reduce radiation exposure, supporting safer paediatric CT imaging.