The perfect pet for a city child; The Magic Guinea Pig, by Tricia Springstubb. New York: Morrow Junior Books. 32 pp. $9.50. (Ages 7-9.)

Many city children today are unable to have the traditional cat or dog, yet they long for a pet of their own. ''The Magic Guinea Pig'' is about such a boy.

Mark wanted a puppy more than anything else, but his father said their apartment was too small, and his mother said puppies make a mess. At the playground he meets a bumbly witch and helps her find a red-eyed beetle. The witch tries to reward him with a puppy, but botches the job, and Mark goes home with a guinea pig - the perfect pet for his situation.

Amusing illustrations by Bari Weissman complement this whimsical story.