'Babyhood' begins on an airplane, with Paul and wife blissfully unencumbered by children. They are seated across from the young parents (graying before his eyes) of a terrorizing 2-year-old and a screeching infant. This sobering reality manages magically to pale in a transcendent moment of the baby's bliss, uncomplicated by drool or colic, and the two decide: Now. Well, more or less now. First they try to get pregnant, making expeditions to the bookstore to case out the shelves of baby books; then there are the bouncy reflections on who is, after all, cut out to parent (I don't know if, for example, Mozart actually had kids, but certainly there is no record of him ever leaving the office early to coach Peewee Soccer League). Later comes the account of sibling rivalry between the newborn and the family dog, and why women make better moms than men. 'Babyhood' is a thought-provoking exploration of the important questions of when and why to have children, many of which are answered in the