Traditionally, babies weighing less than a certain amount (which varies between countries) have been classified as having low birth weight.
Thus, by extension the child mortality rate should be higher among children of smoking mothers.
Both variables are acted on independently by smoking and other adverse conditions—birth weight is lowered and the risk of mortality increases.
Therefore, otherwise healthy babies (who would weigh more if it were not for the fact their mother smoked) are born underweight.
However, they still have a lower mortality rate than children who have other, more severe, medical reasons why they are born underweight.