Even if a child is buckled with the seat belt, the impact can make the child slips through the belt and ejected him or her. I don't simply say so, go through car seat tests in you tube, you'll know what I meant.
Here, I've made an effort to make you understand why SEAT BELT is NOT SAFE ENOUGH for children.
Some might find these photos are distressing. But please keep in mind they used dummy and these pictures are for education purposes only. This is the link from you tube http://www.youtube.com/watch?v=1yUl4dCK1GA&feature=related, but I screen captured few photos for people that is unable to watch in you tube (kes kena block dgn IT dept). Credit to SonnieeS.