Head injuries are the most common severe injuries sustained by pediatric occupants in road traffic crashes. Preventing children from adopting positions that can result in an increased injury risk due to unfavorable interactions with the restraints is fundamental. The objective of this paper was to assess the effect of a head support system (SS) on the lateral position of the head, the vertical position of the sternum and the shoulder belt fit. Thirty pediatric rear-seat passengers were exposed to two 75-minute trials. Volunteers were restrained by a three-point belt and, if needed, used the appropriate child restraint system for their anthropometry (high-back booster, low-back booster, no booster). A case crossover study was designed in which the volunteers used the head support system (SS) during one of the trials, acting as their own controls (No SS) in the other. Compared to the control group, the head support reduced significantly the 90th percentile value of the absolute value of the relative lateral motion of the head, regardless of the restraint used. The system also reduced the maximum downward position of the sternal notch within the low-back booster group. As for the belt fit, the use of the head support improved significantly the position of the shoulder belt on the occupant in the low-back booster and in the no booster groups.