Children with primary immunodeficiency (PID) and secondary antibody deficiency (SAD) often require immunoglobulin replacement therapy due to low plasma immunoglobulin G (IgG) levels and recurrent infections. Existing pharmacokinetic models for immunoglobulin in primary immunodeficiency patients predominantly focus on adults, with limited attention to secondary antibody deficiencies and a lesser emphasis on paediatric populations. A population pharmacokinetic analysis was conducted using NONMEM® (7.5.1) on data from 64 patients, with a median age of 4.08 years (range: 0.06–16.8 years). A two-compartment model with first-order elimination, incorporating both additive and proportional residual error, adequately described the data. Inter-individual variability was modelled on clearance, volume of distribution, and baseline IgG levels, with allometric scaling to a 70 kg body weight applied a priori. The estimated clearance was 0.308 L−1 day−1 70 kg−1 (95% CI: 0.23–0.67), and the volume of distribution was 10.96 L−1 70 kg−1 (95% CI: 5.97–15.79). Patients with SAD exhibited a lower clearance rate of 54% compared to PID patients. Dosing simulations indicated that the recommended SAD dosing regimen maintained therapeutic IgG levels in the simulated population. However, only 44.8% to 51.9% of patients with PID achieved target IgG levels with the standard regimen. Administering a loading dose would improve the probability of maintaining therapeutic IgG levels during the 4-week dosing interval. This study provides insights into immunoglobulin pharmacokinetics in paediatric PID and SAD patients, guiding optimised dosing strategies.