SOMEONE SAYS In the morning, as always, we line up to go into school. Someone says, "Let's leapfrog in." And . . . spring-boink, spring-sproink  . . . we do.  Join this lively group of Asian children as they pretend and imagine their way through the day, becoming ponies that prance, birds that swoop, tigers that slurp, and dreamers who invent a thousand other things to become . . . on another day.   Carole Lexa Schaefer and Pierr Morgan, author and illustrator of The Squiggle, bring together vibrant artwork and onomatopoeic language to celebrate and inspire the wondrous creativity of children.  Illustrated by Pierr Morgan.