LAST DAYS OF SUMMER: Would You Believe … ?

Sure, we all want our children to be smart and confident.

That includes the kids we see on stage.

So the temptation is always there for a writer who’s creating a child character to make him super-smart and sophisticated. That way, the audience will be charmed and say “Hey, what a terrific kid, huh?”

Such an approach, however, only works for a few scenes. Soon you realize that the playwright is the one who’s smart and sophisticated. You don’t believe there could be a kid who talks in this manner.

Not only will you soon not believe 12-year-old Joey Margolis but you also wouldn’t want to know him. When the action starts in 1940, the Brooklyn resident is a big fan of a baseball star who plays for the borough’s arch-rival New York Giants. Joey writes to this Charlie Banks and tells him of his serious medical condition so that the man will take pity on him and pay him a visit.

It’s not true – not true at all. Joey is as healthy as a healthy horse.

Such a ruse can make us smile and forgive the kid because he’s too young to understand that he’s doing the wrong thing. Bookwriter-librettist Steve Kluger takes it too far by having him continue to tell whopper after whopper. Joey just doesn’t, to use that old expression “lie like a rug”; he lies like all the rugs in the Burlington Carpet Warehouse.

He’s cocky when delivering his falsehoods, fully expecting to get away with it because he firmly and uncontestably believes that he’s always, always, always the smartest one in the room. Hence, he’ll set an adult straight while placing a condescending hand on the man’s shoulder.

Joey securely grabs his suspenders halfway down his torso while strutting around the stage with ease. He has a confidence that real kids simply don’t have when they’re dealing with adults.

As a result, Joey is unremittingly brash, smug and obnoxious to the point where you don’t root for him; on the contrary, you want him taken down a peg or two (or seven). When this kid gets to high school, if there’s a club called Future Con-Men of America, look for this little phony to be voted president all four years.

When Charlie does show up in Joey’s neighborhood, you’d expect the kid to be awed. He isn’t for a minute. The matter-of-fact way that Joey responds suggests that the kid never had any doubt that The Great Charlie Banks would make a pilgrimage to see The Greater Joey Margolis.

Joey becomes the Giants’ bat boy, a job that all young baseball fans coveted whenever they watched a game. Yet the thrill of it all – meeting baseball stars, sitting in the dugout and (last but hardly least) getting paid – doesn’t seem to impress Joey at all. We hear a good deal about recent and current generations having an overblown sense of entitlement; Joey seems to be their forebear.

Worse, when Joey’s best friend endures something terrible, the only thing Joey can think about is himself. He doesn’t give a thought to his pal, but is solely concerned on how this turn of events will affect him.

True, Joey eventually sees with his own eyes the ramifications of what’s happened to his buddy; that’s when he seems – SEEMS – to straighten out. So one could read the show as a coming-of-age story where a boy must learn something harsh to become a better adult. But that brings us to a more severe problem.

Kluger is intent on starting the show with an adult Joe (the fine Danny Binstock) who we see hasn’t learned much in the ensuing decades. He establishes in the show’s early moments that he’s holding a grudge against Charlie; when we find out what it is, Joe seems utterly immature and selfish; once again, he’s made the situation all about him.

None of the flaws of Joey’s characterization is remotely the fault of Julian Emile Lerner, the very talented young man who plays him. After he proves throughout Act One that he can sing and deliver dialogue quite well, he gets the chance in the second act to dance; here he shows that he’s conquered that skill, too.

As Charlie, Bobby Conte Thornton is wonderful, too. He conveys the type of Bronx neighborhood kid who played enough playground ball to make it to the major leagues. What a terrific way he has with Joey, talking to the lad in a most sincere fashion.

Kluger is an avowed longtime Broadway observer so his libretto includes a time-honored convention of musical theater: after the happiest moment of the entire show occurs, it’s immediately followed by the direst event imaginable. (You’ll probably think of it before the characters do.)

Because the ‘40s had many volatile moments for many Americans — especially for those who were unjustly perceived as not being American — Kluger wisely brings these issues into play. The way the story turns out is predictable, but it wouldn’t be nearly as much if Kluger hadn’t structured the show as a flashback. Instead, he should just start in 1940 and plow ahead.

This is Kluger’s first attempt at writing lyrics and he emerges as first-class wordsmith. He rhymes and accents his words with impeccable skill and taste. Kluger also knows which lines are the best to underline the sentiments and which are ideal for the jokes.

Director-choreographer Jeff Calhoun has a body filled with musical theater genes. He delivers solid dances in a speedy production. Calhoun knows that when a song comes to an end, actors should put a button on it by making a gesture or sitting down or crossing legs on the final note. All do, and all do it with pin-point perfect precision.

Composer Jason Howland’s contributions are quite impressive. Unlike many contemporary songwriters who find a tune they like and shove it into a show whether or not it fits the time period, Howland is always cognizant that LAST DAYS takes place in the ‘40s. So he gives a bit of big band and some boogie-woogie among a number of entertaining patter songs. Kim Scharnberg knew how to orchestrate them, too; more miraculously, he gets a robust sound from only eight musicians in the pit.

And there IS a pit here. If you’ve been to The George Street Playhouse in the past, you know that it never had one. Now it does, thanks to an extraordinary new building: The New Brunswick Performing Arts Center. LAST DAYS is playing in the 463-seat Elizabeth Ross Johnson Theater, the mainstage that even sports a balcony.

The second space, the 252-seat Arthur Laurents Theater, named for artistic director David Saint’s beloved mentor, is also part of the new complex. But set designer Beowulf Boritt knew he had the larger theater to play with, and gives the show Broadway-worthy production values.

LAST DAYS OF SUMMER will need more than those to make it in New York. Midway through the first act, a kid is apologetic over something he’s written and mourns “It’s just a first draft.” This musical is beyond that point, but if it retains that line and others that pre-teens simply wouldn’t say, it won’t have the success it wants.