Maintenance with isotonic fluids is recommended in children with gastroenteritis and failure of oral rehydration therapy. However, little is known on the short-term effects of the commonly prescribed intravenous solutions on metabolic balance in children. The aim of this study is to report on our experience with normal saline, dextrose-supplemented saline and lactated Ringer solution. Methods: A retrospective analysis from the charts of all previously apparently healthy children with acute gastroenteritis, mild to moderate dehydration and failure of oral rehydration, evaluated between January 2016 and December 2019 at our institution, was performed. Subjects prescribed the above-mentioned maintenance intravenous fluids and with blood testing immediately before starting fluid therapy and 4–6 h later, were eligible. The changes in bicarbonate, ionized sodium, potassium, chloride, anion gap and glucose were investigated. Kruskal–Wallis test with the post-hoc Dunn’s comparison and the Fisher exact test were applied. Results: A total of 134 out of 732 children affected by acute gastroenteritis were included (56 patients were prescribed normal saline, 48 dextrose-supplemented normal saline and 30 lactated Ringer solution). The effect of the three solutions on sodium and potassium was similar. As compared to non-supplemented normal saline (+0.4 (−1.9 – +2.2) mmol/L), dextrose-supplemented normal saline (+1.5 (+0.1 – +4.2) mmol/L) and lactated Ringer (+2.6 (+0.4 – +4.1) mmol/L) solution had a positive effect on plasma bicarbonate. Finally, the influence of dextrose-supplemented saline on blood glucose was different (+1.1 (+0.3 – +2.2) mmol/L) compared to that observed in cases hydrated with non-supplemented saline (−0.4 (−1.2 – +0.3) mmol/L) or lactated Ringer solution (−0.4 (−1.2 – +0.1) mmol/L). Conclusions: This study points out that maintenance intravenous therapies using normal saline, dextrose-supplemented saline or lactated Ringer solution have different effects on metabolic balance. A personalized fluid therapy that takes into account the clinical and biochemical variables is advised.