A fastener assembly for a spinal fixation system that is particularly useful in treatment of pediatric and small-statured patients includes a fastener, an attachment member, and a locking member. The fastener has a lower portion for contacting a bone and an upper portion integral with the lower portion. The upper portion has two open channels. Each channel is configured and dimensioned to receive a portion of a longitudinal or connecting member along its circumference. The attachment member is positionable on the fastener and at least partially covers the channel that receives the longitudinal or connecting member. The locking member is operatively associated with the upper portion of the fastener and secures the attachment member and the longitudinal or connecting member to the fastener.