Background: Idiopathic painful flexible flatfoot (FFF) in childhood can persist into adulthood, yet long-term data on subtalar arthroereisis via the calcaneo-stop (C-Stop) procedure are scarce. We aimed to evaluate clinical and functional outcomes at ≥10 years post-surgery and compare them with age-matched normative values in healthy populations. Methods: We conducted a single-time-point long-term follow-up on a subset of 232 children (age 10–14 years) selected from a retrospective cohort of 494 patients who underwent bilateral anterograde C-Stop between 2010 and 2014. Inclusion required idiopathic symptomatic FFF refractory to conservative care and a minimum 10-year follow-up. At a mean follow-up of 12.1 ± 2.5 years, patients completed the Foot and Ankle Ability Measure (FAAM) and Tegner Activity Scale (TAS). Secondary data included anthropometrics, implant details, accessory procedures, screw removal, and complications. Results: Respondents demonstrated excellent function: FAAM total 98.8 ± 3.7 (range 75–100) with 87.5% achieving the ceiling score; FAAM–ADL 99.3 ± 3.2; FAAM–Sport 98.0 ± 6.4. The mean TAS was 3.7 ± 2.0, with 53% active in sports—72% low-impact, 12% high-impact non-competitive, and 4% competitive. Sex and history of complications produced statistically significant but clinically small differences (<3% on FAAM total; <6 points on subscales). No outcome differences were observed by age or BMI, accessory procedures, or screw removal status. Conclusions: Ten years after C-Stop arthroereisis in childhood, patients exhibit functional scores comparable to normative values, high rates of ceiling effect on FAAM, and a modest level of physical activity predominantly in low-impact sports.