Background/Objectives: Pediatric flexible flatfoot is a common developmental variation, and treatment is generally considered when pain or functional limitations are present. This systematic review evaluated clinical and biomechanical evidence for sensorimotor insoles in children with flexible flatfoot and contextualized these findings alongside evidence for conventional foot orthoses and exercise-based rehabilitation; head-to-head comparative evidence was not assumed when unavailable. Methods: Following PRISMA 2020 guidelines, comprehensive searches were performed across PubMed/MEDLINE, the Cochrane Library, and PubMed Central up to August 2026. Eligible primary studies comprised randomized controlled trials, randomized comparative or sequence trials, prospective comparative or pre-post studies, and longitudinal cohort studies evaluating pain, function, gait, balance, posture, or neuromuscular outcomes. Risk of bias was assessed using RoB 2 and ROBINS-I, and the certainty of the evidence was summarized qualitatively using GRADE. Results: Ten main clinical studies met inclusion criteria, including 5 RCTs and 5 non-randomized studies, with follow-up periods ranging from 6 weeks to 2 years. Both customized supportive orthoses and intrinsic muscle exercises consistently reduced pain and improved function in symptomatic children in the short- to medium-term. Direct randomized evidence for sensorimotor insoles suggests potential functional and neuromuscular effects; however, the limited evidence base does not support firm conclusions about comparative effectiveness or superiority over supportive orthoses. Evidence for permanent structural arch correction was not established. Conclusions: In symptomatic pediatric flexible flatfoot, conservative treatment should prioritize pain and function. Conventional supportive orthoses and targeted exercise have a broader evidence base, whereas current evidence is insufficient to establish the superiority of sensorimotor insoles.