Wild-haired children and an unpredictable rhino; Who Wants a Cheap Rhinoceros? Text and illustrations by Shel Silverstein. (A revised and expanded edition.) New York: Macmillan. Pages unnumbered. $8.95.

Shel Silverstein fans, take notice! ''Who Wants a Cheap Rhinoceros?'' is a question asked by children who find their own big pal so valuable they'd never give him up. Perhaps a more apt title would be ''Twenty-five fun things to do with a live rhinoceros.'' These charmingly portrayed, wild-haired children have a grand time putting the rhinoceros to work around the house. His horn makes an ideal coat hanger, back scratcher, and doughnut holder.

However, the kids admit, ''He's not too great at opening doors.'' The rhinoceros amiably puts up with it all. More than just a beast of burden, he'll kick up his heels (rhinopods?) at jumping rope and playing pirate. With no sense of his great girth, he'll be just as comfortable sitting on the children's laps as having them sit on his.

Adult readers may cringe slightly at his habit of threatening parents into doling out more allowance and chomping bad report cards on command, but overall he's a kind and spunky book companion, the kind most kids love.

(All ages.)