Observant comedy of a children's world - played by adults

Found a Peanut. Play by Donald Margulies. Directed by Claudia Weill. ''Found a Peanut,'' at the Public/Anspacher Theater, recalls a children's hour from a children's world - but with a difference. The children assembled by playwright Donald Margulies for this touching and sympathetically observant comedy are played by young adults. The device adds perspective to the view and extends (even doubles) the sense of make-believe. This is the way it was, or the way it has been remembered, on the last day of summer in 1962.

The place is the back of a Brooklyn apartment building. An impersonally windowed gray brick wall rises above the square of concrete that serves as a play space for half a dozen children. As the children come and go, squabble and reconcile, bury a dead bird, unearth a buried treasure, and otherwise while away this forlorn Sunday, Mr. Margulies observes the patterns of behavior that differentiate the playmates and at the same time form their common bond.

Although the discovery of the hidden money leads to the play's brief scenes of violence, ''Found a Peanut'' is more concerned with character and incident than with plot.

Never overstating his case, Mr. Margulies intimates the ways in which the child may become the father to the man and mother to the woman. The only innocent in the backyard group is Little Earl, a five year old who greets almost every occasion with unalloyed delight and curiosity. He is enchantingly played by Peter MacNicol. The group is completed by 12-year-old Scott (Greg Germann), who throws his weight and his age around until a couple of intruding bullies (Johnathan Walker and Kevin Geer) invade the scene to taunt and terrorize the youngsters.

The performance staged by Claudia Weill is more than just juvenile impersonation a la Lily Tomlin (clever as Miss Tomlin is). By their skills of characterization, the cast members of ''Found a Peanut'' create the illusion of a juvenile world, draw the spectators into it, and enlist our sympathies for these children apprehensively facing their next step into a kind of unknown - the opening of school on Monday. In its short 70 minutes (without intermission) this backward look at a group of city kids becomes an involving theater experience.

Thomas Lynch designed the grubby backyard, with its hostile brick faCade. Beverly Emmons lighted the production and Jane Greenwood costumed the youngsters. B. H. Barry directed the quite terrifying invasion by the bullies. ''Found a Peanut'' is more than child's play.