Dehydration is common in children for physiological and behavioral reasons. The objective of this study was to assess changes in hydration status and renal impairment across school weekdays. We conducted a longitudinal study of three repeated measures of urinalysis within one week in November 2019 in a child cohort in Beijing, China. We measured urine specific gravity (USG) to determine the dehydration status, and the concentration of beta(2)-microglobulin (beta(2)-MG) and microalbumin (MA) to assess renal function impairment among 1885 children with a mean age of 7.7 years old. The prevalence of dehydration was 61.9%, which was significantly higher in boys (64.3%). Using chi-square tests and linear mixed-effects regression models, we documented the trends of the renal indicators' change over time among different hydration statuses. Compared to Mondays, there were apparent increases of beta(2)-MG concentrations on Wednesdays (beta = 0.029, p < 0.001) and Fridays (beta = 0.035, p < 0.001) in the dehydrated group, but not in the euhydrated group. As for the MA concentrations, only the decrease on Fridays (beta = -1.822, p = 0.01) was significant in the euhydrated group. An increased trend of elevated beta(2)-MG concentration was shown in both the euhydrated group (Z = -3.33, p < 0.001) and the dehydrated group (Z = -8.82, p < 0.001). By contrast, there was a decreased trend of elevated MA concentrations in the euhydrated group (Z = 3.59, p < 0.001) but not in the dehydrated group. A new indicator ratio, beta(2)-MG/MA, validated the consistent trends of renal function impairment in children with dehydration. Renal impairment trends worsened as a function of school days during the week and the dehydration status aggravated renal impairment during childhood across school weekdays, especially tubular abnormalities in children.