Parental willingness is a prerequisite for childhood immunization. This study evaluated whether parental vaccination readiness and willingness, measured using the modified Japanese 7C Vaccination Readiness Scale, predicted vaccination behavior. In December 2021, 2 months before the approval of COVID-19 vaccines for children aged 5–11 years in Japan, 1190 parents of children in this age group were surveyed regarding their intention to vaccinate their children against COVID-19. The survey utilized an online version of the modified 7C Vaccination Readiness Scale, comprising seven components: Confidence, Complacency, Constraints, Calculation, Collective Responsibility, Compliance, and Conspiracy. The modified 7C components were analyzed for model fit and their association with parental intention using structural equation modeling. The scale demonstrated good model fit (goodness-of-fit index: 0.98), with high standardized factor loadings for parental readiness (factor loadings: 0.84). In January 2024, the same parents were surveyed about their children’s COVID-19 vaccination status (valid responses: 536). The relationship between the components of the modified 7C scale and actual vaccination behavior was examined. Parents with low readiness to vaccinate their children against COVID-19 exhibited a significantly higher non-vaccination rate. These findings suggest that the modified 7C scale could support the development of targeted interventions to improve childhood vaccination uptake.