In this frigid, ponderous rendering of the familiar rhyme, three youngsters rig up a box-boat in an attic . . . which then materializes into a cloud-borne craft outside . . . and finally carries them back . . . to be discovered, sleeping, by their parents. The children are pallid; the colors are uniformly, deliberately drab; the whole has indeed an unhealthy, almost spectral appearance--but not a jot of magic. Both the make-believe and the sentimentality are better served by Barbara Cooney's moon-lit transfiguration.