The paper comprises the most important observations concerning the appropriate use of child car seats, with special attention being given to seat belt straps, which are among the critical elements of the child restraint system. The appropriate positioning and pre-tensioning of the child restraining straps and the most frequent faults in using the belts have been highlighted. Results of strength tests of belt straps of 25.4×1.56 mm and 38.6×1.23 mm cross-section (narrow belts and wide belts, respectively) have been presented. The time histories and distribution of forces in the shoulder and lap belts restraining a child with a mass of 15 kg were examined. Two types of child safety seats, referred to as “safety seat 1” and “safety seat 2”, were used. A car crash was simulated by braking a measuring trolley from a velocity of 50–52 km with a deceleration of up to 28 g. The distribution of forces in the shoulder and hip belts of the harness restraining a test dummy was analysed. The force distribution was found to depend on the seat construction. The test results and graphs presented indicate a possibility of evaluating the effectiveness of child safety seats in the car collision phase.