Background: Proper fluid and electrolyte balance is vital for preventing morbidity in neonates and children; thus, this study aimed to assess and compare electrolyte imbalances and renal marker profiles between newborns and children. Aim of the study: The aim of the study was to assess and compare electrolyte imbalances and renal marker profiles between newborns and children. Methods: This cross-sectional study at the Department of Biochemistry and Molecular Biology, Bangladesh Shishu Hospital & Institute, Dhaka (Jan–Mar 2024), included 100 participants (50 newborns, 50 children). Serum creatinine, blood urea, and electrolytes (Na⁺, K⁺, Cl⁻, Ca²⁺) were measured, and data analyzed with SPSS 26 using t-tests, chi-square/Fisher’s exact tests, and Pearson’s correlation (p < 0.05). Results: Newborns had higher creatinine (0.85 vs. 0.47 mg/dL, p = 0.0009) and urea (78.3 vs. 29.5 mg/dL, p = 0.012) than children. Electrolyte imbalance occurred in 58% (64% newborns, 52% children), mainly hypernatremia (24%), hypokalemia (18%), and hyperkalemia (15%). Newborns showed lower sodium (139.8 vs. 146.1 mmol/L, p = 0.037) and chloride (100.8 vs. 106.5 mmol/L, p = 0.016), but higher potassium (5.1 vs. 4.3 mmol/L, p = 0.030). Creatinine and urea correlated negatively with sodium (r = –0.32, –0.24) and positively with potassium (r = 0.68, 0.41; all p < 0.05). Conclusion: Electrolyte imbalances are common in newborns and children, with renal dysfunction closely associated with sodium and potassium disturbances, highlighting the need for age-specific monitoring.