This study aimed to investigate the clinico-radiological factors of related future surgical treatment in patients with pediatric flexible flatfoot (FFF) who first visited the orthopedic clinic before the age of 10 years. Sixty-five patients diagnosed with moderate/severe idiopathic FFF deformity between the ages of 2-10 years were included. We developed prognostic models for the risk of the surgery during the follow-up period. Twenty (30.8%) patients required surgical treatment, and all of them underwent calcaneal lengthening osteotomy. Among them, 7 (10.8%) patients required concomitant Achilles-lengthening surgery. Unilateral involvement, older age, smaller calcaneal pitch angle, and higher talo-first metatarsal malalignment were considered high-risk factors for pediatric FFF surgery. Prognostic models identified three prognostic risk groups based on those factors, and survival curves revealed significant differences among the groups. Our prognostic models help predict the failure risk of conservative management of pediatric idiopathic FFF. Level of Evidence: Level III, prognostic study.