If only I could be ...; The Luckiest One of All, by Bill Peet. Boston: Houghton Mifflin Company. 30 pp. $9.95. (Ages 4-7.)

A little boy sitting up in a sycamore tree decides it would be much more fun to be a bird, flying high over the treetops. But a brown-speckled thrush disagrees, because he is always caught up in a last-minute rush, ''chasing about in the stormiest weather, drenched and bedraggled to the very last feather.'' If given a choice, the thrush would trade places with a lazy old fish.

From here, this enchanting picture book by Bill Peet takes off in rhythmic verse, and the reader discovers the advantages and disadvantages of being a turtle, frog, lion, gopher, steam shovel, streetlamp, lighthouse, and more.

The illustrations accompanying each verse are colorful and amusing, as each of the objects takes on human characteristics. My favorite is a forlorn-looking lighthouse, with a stormy ocean lapping at its feet, and its head surrounded by ''swirling fog and weird atmosphere.''

It's the comfortable cat, curled up in a soft easy chair, who closes the story. He spends his days in such feline activities as strolling through a garden, climbing a peach tree, and taking catnaps, but yearns to do more exciting things. He wishes he were a small boy, because of all the things a boy can do - ride a bike, climb a tree, fly a kite, play football, and even read a good book - of which this would be an excellent choice.