Craftily structured, this is Posy's bedtime--the three stories (and an encore) that she wangles from her father about when she ""was little."" (Age three, say, to her present six or seven.) ""Posy and the Sheets"" has Mommy going out to the fish store, leaving Daddy to order four pairs of new sheets--any kind or color, ""That's when you looked up at me and said PINK."" With Posy continuing to say PINK, the family winds up with one pair of plain pink sheets, one pair of pink sheets with pink stripes, one pair with pink flowers and one pair with pink fish. ""So that's why. . . ,"" says today's older Posy--who hates pink. ""Posy and the Nighttime,"" though less original, has its own sly, present-day echo: Daddy and Mommy, she knows, ""still do"" kiss her good night when they come home after she's asleep. ""Posy and the Pocketbook,"" contrastingly, doesn't end with a come-back: the first time Posy went to the store with money, she turned shy--and she's never forgotten it. The last, just-one-more story is really a recollection of wishes. Does Posy remember? ""I just needed to be reminded."" Light years away from the usual when-you-were-little endearments--but then Pomerantz (The Tamarindo Puppy, Buffy and Albert) can be counted upon for the unusual. Catherine Stock's pastel-shadowed pencil drawings are gentle, amusing, oblique--too.