BACKGROUND Self-reporting is the gold standard for measuring pain in adult pain management; however, this issue is unique and different in children. Accurate pain assessment for the appropriate management of children's pain is important. OBJECTIVE This study was conducted to assess the pain level by the child, mother, and nurse during intravenous line insertion in preschool children, candidates for surgery in Tabriz Children's Hospital. METHODS This is descriptive observational study. A total of 140 eligible children were included in the study using convenience and sequential methods. The intravenous line was inserted by an experienced nurse and pain assessment was performed by the child, mother, and nurse immediately after procedure, using the Wong-Baker face scale (WBFS). The data were analyzed through descriptive statistics and repeated measures ANOVA test using SPSS version 13 software. RESULTS The difference between child-mother and nurse-mother mean scores was not statistically significant; however, there was a statistically significant difference between the score measured by the child and the nurse (P = 0.017). CONCLUSIONS The children's pain assessment score during venipuncture was higher than that of mothers and nurses. The mothers' score was more closely related to the children's score. As a result, a mother's assessment can be considered a reliable estimation of proper pain management in young children and mothers can act as a moderator and actualize the nurses' score. Consequently, establishing a three-way communication between mother, child, and nurses is necessary to precisely estimate the child's actual pain and take appropriate measures to reduce it.