What happened to that sunny outgoing child of eight?  As parents of nine-year-olds often discover, nine is a tricky age.   Children are more distant from Mother and Father; they're more independant and rely on friends for companionship, or they have a tendency to spend time alone.   Some nines are boisterous and wild, others thoughtful and withdrawn.   Helping parents learn how to cope with the unpredictable nine-year-old is the aim of this practical guide from the Gesell Institute. Nine-year-olds are hovering on the brink of adolescence, and this in part contributes to their up-and-down nature.   Dr. Louis Bates Ames and Carol Chase Haber paint a vivid picture of the child at this age and offer useful advice to make life easier for parents and children alike.