In “Bumping Into Mr. Ravioli,” Adam Gopnik’s marvelous 2003 “there but for the grace of God goes my family” essay in the New Yorker, Mr. Gopnik’s young daughter has a hard time fitting into her imaginary friend’s schedule. Olivia needs to clear any plans through Mr. Ravioli’s (also imaginary) personal assistant, a situation that causes no end of hand-wringing over the relentless pace of Manhattan and the business of busyness.

Picture Neil LaBute turning his malevolently empathic eye on “Bumping Into Mr. Ravioli,” and you’ll get a sense of the wicked social commentary on display in Noah Haidle’s “Mr. Marmalade,” currently spurring fits of anxious laughter at the Roundabout Theatre Company’s smaller Laura Pels space. The play’s certainly not for all tastes, and it coasts on shock value occasionally, but Mr. Haidle’s brutal wit and sharp ear for dialogue mark him as a provocative young talent.

With no siblings, no dad in sight, and a distracted slattern for a mom, little Lucy (Mamie Gummer) has conjured up what’s known in child-psychology circles as a “paracosm” – a fully realized imaginary society. But this world doesn’t appear to be much of an improvement on the real one. Lucy is a full year older than Olivia was, and apparently a lot can happen between the ages of 3 and 4. She’s old enough to confront the dapper Mr. Marmalade (Michael C. Hall of “Six Feet Under” fame) over his possible infidelities and young enough to make him “pinky-swear” his denials.

Pretty soon, Mr. Marmalade’s assistant, Bradley (a terrific David Costabile), is wearing sunglasses to cover a black eye and making lame excuses about not picking up his boss’s dry-cleaning. It’s not unusual for children to receive apologies from their imaginary friends, but the apology typically doesn’t come as part of a 12-step program. No wonder Lucy warms up to the idea of playing doctor with 5-year-old Larry (Pablo Schreiber), despite his being the youngest suicide attempt in the history of New Jersey. (“The nurses took my picture and hung it in the lobby.”)

It seems we can’t get enough of children acting like grown-ups, from the ubiquitous lisping sages of sitcoms to that grotesque talking toddler in the Quizno’s ads. But this trope depends on a sort of harmless impishness: Preschoolers are supposed to say the darnedest things, not slit their wrists and get punched.

Director Michael Greif tempts fate by having Ms. Gummer and Mr. Schreiber – both of whom are at least two decades older than their characters – play up their childishness in the middle of all the psychological violence. Their clothes are disastrously mismatched; their legs constantly fidget. It shouldn’t work, but the actors weather their characters’ childlike whims and jaded misery with equal fearlessness.

Roundabout ran into some trouble by casting Mr. Hall’s “Six Feet Under” brother, Peter Krause, in last summer’s misguided revival of “After the Fall,” but the same mistakes are not repeated here. Mr. Hall’s risky, borderline hysterical performance nails a child’s exaggerated notion of how a nice guy (and a not-so-nice guy) would look and act. Not once does he attempt to curry favor with the audience, which makes him that much more endearing.

Mr. Haidle succumbs to impishness himself now and then, as when Lucy and Larry exchange quips about U.S. health care. And he has a tendency to ladle on the nihilistic irony, a habit that Mr. Greif neglects to rein in. The kitschy music by the usually dependable Michael Friedman is one example, as are the precious subtitles to each scene: “III. Concerning Countless More Hardships Which Lucy Endured With Regard to Her Imaginary Friends, If You Can Even Call Them That.”

In between the signposts, however, Mr. Haidle depicts the dangers of an active imagination while demonstrating the dark fecundity of his own. (He has also cracked the long-elusive problem of how to stage a food fight on stage without leaving a horrible mess for the rest of the play.) On two different occasions, Lucy says of her situation, “It’s complicated,” to which someone responds, “No, it’s simple.”

Being a child is complicated, a fact adults often forget. Luckily, Mr. Haidle is young enough to remember – and skilled enough to remind us with both cheek and charm.

***

“Mr. Marmalade” marks yet another entry into a mini-youth movement by the major off-Broadway theaters. Within the last three months, five have mounted productions by new writers well under 40: the Atlantic (Rolin Jones’s “The Intelligent Design of Jenny Chow”), Manhattan Class Company (Laura Wade’s “Colder Than Here”), New York Theatre Workshop (Itamar Moses’s “Bach at Leipzig”), Second Stage (Dan O’Brien’s “The Dear Boy”), the Public Theater (Rinne Groff’s “The Ruby Sunrise”), and now the Roundabout. Manhattan Theatre Club and Lincoln Center jump on board next year with works by fellow darlings Roberto Aguirre-Sacasa and Sarah Ruhl, respectively.

Some of these plays have been markedly better than others, but every one was superior to many works by far more established playwrights mounted off-Broadway in the very recent past. Maybe the old boys’ (and girls’) club will be back soon, but in the meantime, the youngsters have earned our support.

Until January 29 (111 W. 46th Street, between Sixth and Seventh Avenues, 212-719-1300).