Stella and Roy

In the park one Sunday, Stella challenges her younger brother to a race around the lake. Given that she is riding a big-wheel trike while Roy's toddler-style four-wheeler is pushed rather than peddled (``It was built for coasting,'' the narrator explains), Stella's victory seems certain. But the curious girl is distracted by several natural displays, among them an assembly of turtles, gulls and a frog; a group of fish in a pool below a waterfall; a gaggle of friendly geese; and a policewoman's horse. Roy wins the race, but he shares his prize--a box of popcorn--with his sister. Far more impressive than Wolff's ( A Year of Birds ; Come with Me ) pleasant though somewhat thin plot line are her polished linoleum block prints, hand-tinted with watercolor. Featuring the fresh, earthy colors of spring, these distinctive illustrations convey both the motion of the children and the still beauty of the park. Ages 2-5. (June)