It would take Charlotte Pomerantz to think of giving young Daniel the idea of giving ""a half-birthday party"" for his six-month-old sister Katie, who has just stood for the very first time. The guests, moreover, are supposed to bring half a present; but ""You have to tell a whole story about the half present."" The presents and stories are variously simple and ingenious, but all are at least diverting. Daniel's friend Lily brings one slipper, because her dog ate the other half of the pair; Daniel's mother brings one earring (to be put away, till Katie is grown) because she lost the other when she met father. Grandma and her friend Mr. Bangs each bring half the birthday cake. Father presents Kate with an unfinished poem--wondering, as everyone now is, ""What Daniel will bring. . . ?"" Daniel, who'd forgotten about the present, has been wondering too. . . and then he takes everyone out on the fire escape, to see a gleaming half moon. (""Is it true,"" asks his mother, ""that you knew all along that you would give Katie a half-moon?"" ""It's half-true,"" says Daniel.) The illustrations are serviceable if undistinguished cartoons (in Quackenbush et al. pink and orange colorations); the story has the episodic structuring and plot-loops of a beginning reader--a function this could agreeably perform altogether.