DREAM CHILDREN by Gail Godwin (Ballantine: $12, 242 pp.). Godwin's short stories suggest museum display cases, where her characters exhibit their vulnerabilities. A married man and his mistress face separation with very different attitudes in "My Lover, His Summer Vacation"; a seemingly content housewife calmly plans a murder-suicide in "Interstices." Mrs. Wakeley in "Nobody's Home" watches her husband and muses, "Is that what marriage is, after all: details? The gradual accumulation of details blocking up the person you once mysteriously loved, until that person can no longer be seen?"