Background: Ceftolozane-tazobactam is a new antibiotic against multidrug-resistant pathogens such as Pseudomonas aeruginosas. Ceftolozane-tazobactam dosage is still uncertain in children, especially in those with renal impairment or undergoing continuous renal replacement therapy (CRRT). Methods: Evaluation of different ceftolozane-tazobactam dosing regimens in three critically ill children. Ceftolozane pharmacokinetics (PK) were characterized by obtaining the patient’s specific parameters by Bayesian estimation based on a population PK model. The clearance (CL) in patient C undergoing CRRT was estimated using the prefilter, postfilter, and ultrafiltrate concentrations simultaneously. Variables such as blood, dialysate, replacement, and ultrafiltrate flow rates, and hematocrit were integrated in the model. All PK analyses were performed using NONMEM v.7.4. Results: Patient A (8 months of age, 8.7 kg) with normal renal function received 40 mg/kg every 6 h: renal clearance (CLR) was 0.88 L/h; volume of distribution (Vd) Vd1 = 3.45 L, Vd2 = 0.942 L; terminal halflife (t1/2,β) = 3.51 h, dosing interval area under the drug concentration vs. time curve at steady-state (AUCτ,SS) 397.73 mg × h × L−1. Patient B (19 months of age, 11 kg) with eGFR of 22 mL/min/1.73 m2 received 36 mg/kg every 8 h: CLR = 0.27 L/h; Vd1 = 1.13 L; Vd2 = 1.36; t1/2,β = 6.62 h; AUCSS 1481.48 mg × h × L−1. Patient C (9 months of age, 5.8 kg), with severe renal impairment undergoing CRRT received 30 mg/kg every 8 h: renal replacement therapy clearance (CLRRT) 0.39 L/h; Vd1 = 0.74 L; Vd2= 1.17; t 1/2,β = 3.51 h; AUCτ,SS 448.72 mg × h × L−1. No adverse effects attributable to antibiotic treatment were observed. Conclusions: Our results suggest that a dose of 35 mg/kg every 8 h can be appropriate in critically ill septic children with multi-drug resistance Pseudomonas aeruginosa infections. A lower dose of 10 mg/kg every 8 h could be considered for children with severe AKI. For patients with CRRT and a high effluent rate, a dose of 30 mg/kg every 8 h can be considered.