Flexible intramedullary nail fixation provides excellent fixation in children with unstable tibial shaft fractures, but few published series demonstrate the results and complications with this technique in children. A retrospective review of 19 patients was performed, as well as a biomechanical analysis of two implant configurations. Outcome measures included union rates, residual deformity, and complications. Union occurred in all cases. Five patients (26%) had complications. None required repeat operation. Two (11%) angular deformities (>/=10 degrees) occurred with the medial C and S construct, versus none with the double C. The C and S construct was more stable to mechanical testing with axial and torsional loading. Flexible intramedullary nail fixation is a straightforward technique that reliably produces good results. While the C and S construct was superior in biomechanical testing, the double C construct is more reliable and straightforward and remains by far the authors' preferred technique.