In sprightly ballad stanzas, two children fend off a cheerful, childishly importunate Elijah, who has arrived too early in the Passover Seder."" 'Let me in!' he calls to us,/'I'm ready! I'm all set!'/And pulls his whiskers when we say,/'Elijah, please, not yet!' "" As the Seder progresses, poor Elijah grows more and more impatient (""Of all the plagues that we recite. . .I think that he must surely love/The frogs the very best./Because he hops around the porch. . .""). But after Elijah finally enters; the roles are reversed: the children beg him not to go--""Elijah, please, not yet!"" Halpern, who is remembered for Barbara Cohen's The Carp in the Bathtub, contributes fine-line b&w drawings showing the prophet as heroically large but transparent, nicely capturing the story's comic spirit. An amusing new angle on the Seder meal.