A model of a built-in children restraint system was set up. The method of coupling between MADYMO and LS-DYNA was used in simulation and a sled test was adopted to validate this model. First, side impact simulations were carried out respectively for vehicle seat without armrest, with different types of armrests and the combination of armrest and lateral flank. Then, an analysis of the influence of the different heights of armrest for children was done through other side impact simulations. Finally, the simulation results were analyzed. The results show that the armrest can prevent the child occupant from slipping out of the seat cushion so as to avoid suffocation death caused by neck bridled, while lateral flank can protect children's neck and chest effectively. Vehicle seat with the combination of lateral flank and armrest can provide the best protection for children in all types. The results also show that the seat with moderate armrest height can provide the best protection for children.