Stella is afraid of the dark,"" says a freckled tyke, pointing to his drowsing, obviously unflustered dog. And Stella never moves as, one by one, he recounts her supposed fears. His listening friend, a somewhat bigger (black) girl, finally pipes up: ""Stella is not very smart, is she?"" And when he grudgingly acknowledges that ""Stella IS silly,"" she advises hugs and other such reassurances--which, in the final wordless sequence, we see Stella administering to her quaking master. Not a novel situation, perhaps, but deftly executed at this very-easy-reading level, with entertaining dialogue and pictures that clearly--and indulgently--tell a different story.