Purpose. To establish the early outcome, satisfaction and complications of sinus tarsi implants in the management of symptomatic flexible flatfeet for a paediatric population. Methods and results. We included all patients aged 18 years or less who were treated for flexible flatfeet with a sinus tarsi implant between January 2010 and June 2012. We excluded patients who had a history of clubfeet or tarsal coalition. 34 patients had 59 implants. The mean age at surgery was 13.7 yrs (9–17 yrs), with mean follow-up of 22 months (range 10–35). Mean AOFAS improved from 65.7 to 87.9 (p<0.001), with an improvement in AOFAS pain scores (p=0.0001). Radiographic correction occurred in all feet, with average improvement of the anteroposterior talar-second metatarsal angle of 16 degrees, and the lateral talar-first metatarsal angle of 9 degrees. Implant placement satisfaction rate was 86%, with 81% claiming that they would have the procedure again. Complications included peroneal spasm (8%), extrusion (7%), revision (5%), and removal (5%). Peroneal spasm was recalcitrant in 3 of the 5 cases, and those with spasm had a higher average forefoot abduction measurement (35 degrees) than the remaining treated cases (25 degrees). Conclusion. In paediatric patients with flexible flatfeet, utilisation of the sinus tarsi implant has good short-term outcome, with our failure and revision rates comparable to other published studies in the adult population. We introduce the concept of isolated subtalar hyperlaxity which appeared to be a distinct etiological factor. A learning curve in assessing implant size was noted. Overcorrection and implant migration were the indications for revision surgery. Severe forefoot abduction appears to have a poorer outcome, and may be associated with peroneal spasm. Level of evidence: IV