Low cardiac output is one of the most common complications after cardiac surgery. Levosimendan, a new inotrope agent, has been demonstrated in adult patient to be an effective treatment for this purpose when classical therapy is not effective. The aim of this study was to evaluate the effect of Levosimendan on cardiac output parameters in cardiac children with low cardiac output syndrome (LCOS.). We carried out a retrospective analysis on 62 children hospitalized in our pediatric intensive care unit (PICU) after cardiac surgery, which demonstrated LCOS not responding to classical catecholamine therapy and who received levosimendan as rescue therapy. LCOS parameters like diuresis, central venous oxygen saturation (SvO2), venous-to-arterial CO2 difference (∆avCO2), and plasmatic lactate were compared before therapy and at 3, 6, 12, and 24 h after the beginning of the levosimendan infusion. We also analyzed the effect on the Vasoactive-inotropic score (VIS), adverse events, and mortality. After the beginning of levosimendan infusion, diuresis (1.1 vs. 3.5 ml/kg/h, p = 0.001) and SvO2 (59.5 vs. 63.3%, p = 0.026) increased significantly during the 24 h of infusion, and at the same time, plasmatic lactate (2.3 vs. 1.3 mmol/l, p < 0.001) decreased. ∆avCO2 (10.8 vs. 9.4 mmHg, p = 0.21) and the VIS (44.5 vs. 22.5, p = 0.143) also decreased, but not significantly. No side effects were noted. The mortality in this patient group was 16%. Levosimendan is an effective treatment in children presenting LCOS after congenital heart surgery. Our study confirms the improvement of cardiac output already shown in other pediatric studies, with no undesirable side effects.