It acts like the pill, which means it can stop ovulation, stop fertilization, or stop implantation. The bottom line is that it prevents pregnancy. If, despite taking it, a woman gets pregnant anyway, the pregnancy will be normal. Unless she decides to kill the kid...
I agree, Plan B should not be sold to children. It is a stronger drug than the pill, and may be safe to take once or twice. However, there almost certainly will be some who will take it every time they have intercourse. It is not intended to be taken that way, and I don’t think that kind of usage is safe.