The aim of this study was to evaluate the ability of parents to estimate the weight status of preschool children attending the Instituto Mexicano del Seguro Social (IMSS) day-care centers using three different tools. A total of 100 parent-child pairs attending to all the existing IMSS day-care centers in Ensenada (n = 9) completed a questionnaire on the perception of children's weight status using verbal description, sketches, and pictures. Chi squared test and univariate logistic regression was applied to assess the difference in perception between the tools used, the factors associated with the weight status perception and to identify predictors of parental underestimation of their child's weight. The sample size was estimated for a significance level of 0.05 with statistical power of 80%. No significant differences were found in the perception of weight status using different tools. The parents' underestimation of the child's weight status ranged from 51 to 59%, this percentage jumps to 79 to 84% in overweight children and 82 to 91% in obese children. Being a young mother and having a daughter increased the risk of underestimation. Higher odds of underestimation were found in > 2BMI z-score. The high underestimation found in this study shows that the ability of parents in signaling an alert to prevent childhood obesity might be highly reduced and preventive health programs should include increasing the weight status perception.