Background: Flatfoot is a deformity in which the foot is flattened due to a decrease in or loss of the medial longitudinal arch. Statement of the problem: Few studies have investigated the relationship between the severity of flat feet, trunk strength, and joint flexibility. Purpose: The aim of this study is to investigate the relationship between the severity of flatfoot and joint flexibility and foot and trunk strength in children with flexible flatfoot. Methods: This study included 16 children (boys, 12; girls, 4; age, 4~8 years) with flexible flatfeet. We examined the resting calcaneal stance position angle (RCSPA) and foot posture index (FPI) scores for clinical severity and radiographic parameters, such as calcaneal pitch angle, talometatarsal angle (TMA), and talocalcaneal angle (TCA). Muscle thicknesses of the tibialis posterior (TP), peroneus longus (PL), and L1 multifidus were measured by sonography. Isometric contraction of ankle inversion, eversion in a seating position, and lumbar extension at a prone position were induced using a handheld dynamometer to measure the maximum muscle strength for each muscle. Beighton’s scoring system was used to assess joint flexibility by evaluating the hyperextension of the joint for each category when performing stretching motion. Spearman’s rank correlation coefficient for nonparametric data was used. Results: The FPI showed a moderately negative correlation with the muscle thickness of TP (r = −0.558, p = 0.009) and L1 multifidus (r = −0.527, p = 0.012), and the strength of the ankle inverter (r = −0.580 p = 0.005) and lumbar extensor (r = −0.436 p = 0.043). RCSPA showed a moderately positive correlation with TCA (r = 0.510, p = 0.006). Beighton’s score showed no significant correlation with all parameters. Conclusion: In children with flatfoot, FPI reflected the clinical severity; thus, the more severe the symptoms, the weaker the ankle inverter and lumbar extensor.