The investigator will randomly divide pediatric patients aged less than 7 years old into crystalloid and colloid groups. Crystalloid group receive only crystalloid, and colloid group receive hydroxyethyl starch (HES, 130/0.4). The maximum volume of HES is limited less than 30 ml/kg. The same transfusion protocol will be applied in both groups. From the day of operation to postoperative 7 days, glomerular filtration rate (GFR), serum creatinin level (Cr) will be checked. Acute kidney injury (AKI) is defined when there is more than 50% increase in Cr when compared to baseline level. The investigator will compare the clinical outcomes including AKI between two groups.