In this paper we study the control of a lower limb exoskeleton dedicated for Kid’s rehabilitation. A fractional order proportional-derivative (FOPD) control with constant gravity compensation is proposed for the regulation of a robot with two joints. The control law requires the measure of the position and the used torque of the two joints. The goal is to stabilize the robot in a desired trajectory with less error and energy.A simulation study reveals the better performance that can be obtained with the fractional proportional derivative with gravity compensation as compared to an FOPD for the first performance criteria (the IAE) and the PD with gravity compensation for the second criteria (the IAU).