Cast of kids in a smart new musical; 'Really Rosie' Presented by Chelsea Theater. Book, lyrics, and production design by Maurice Sendak. Music by Carole King. Directed and choreographed by Patricia Birch.

Rosie's natural habitat is Brooklyn. But she has the gift of make-believe, so she sees her neighborhood as it ought to be, not as it really is. That's how drab Avenue P becomes the backdrop for an exciting -- and wholly imaginary -- new "movie" about that fabulous new star, Rosie herself. The toast of the block, if not of the town, she pours out her story to invisible reporters , and enlists the other neighborhood kids to play parts in her upcoming epic.

Did I mention that Rosie is just a kid herself? Yet there's nothing pint-sized about her fantasy life. It's big enough to include us all, and to provide the makings for this smart new musical at the Chelsea Theater Center. It was concocted by grown-up Rosies named Maurice Sendak, Carole King, and Patricia Birch. And it's performed by an all-kid cast -- who, with an average height of something like 4 ft., pack more square inches of sheer talent than many full- sized entertainers.

In many ways, "Really rosie" is a risky undertaking for the Chelsea. There's not a single adult in the show, aside from a couple of off-stage voices, hurling motherly threats at their tardy children from the sidelines. And there are times wen the atmosphere leans toward that most cloying of genres, the cute-kid revue, wherein the performers rely less on their own gifts than on the bustling execution of orders from their elders.

Most of the way, though, "Really Rosie" works really well. Much of the credit goes to Sendak, who wrote the book and lyrics, and designed the decor. His characters are real, recognizable kids -- the bookworm, the tough guy, the poky little brother -- touched with just enough dramishness to lift them a smidgen above ordinary reality.

Their songs and dances are also about real, recognizable situations: the musical numbers have such evocative titles as "Screaming and Yelling," "Simple Humble Neighborhood," and "The Awful Truth." Sendak even elevates the meekest of kiddie conventions to new heights. The alphabet song and the number song take on exciting rensonances in "Aligators All Around" and "One Was Johnny," the latter about a boy "who lived all alone, and liked it that way."

True, the show rarely approaches the exquisite, almost mythic power that Sendak reaches in his best books, such as "Where The Wild Things Are" and "Higgledy Piggledy, Pop." For example, there's still a lot of charm in the episode about Pierre, a boy who "doesn't care" about anything, until a lion eats him up, which changes his attitude mighty quick, especially after he's rescued. But acted out before your eyes, with bouncy music and eager kids tearing around the stage, it seems like just a cute story. The cutting edge -- the sense that this would be really unsettling if it weren't so funny and so emotionally true -- doesn't quite come across.

Still in all, the show has an enthusiasm and an inventiveness that carry it through triumphantly, at least until the last scenes, when the structure begins to sag, and it feels like material is being tacked on for no better reason than giving us your money's worth. This is still the Sendak whose heroine of "Higgledy Piggledy, Pop," sought meaning in life by joining the Mother Goose World Theater. His glee in theatrics is shared by Carole King, whose score is so lively you can't sit still most of the time, and Patricia Birch, whose direction and choreography charge ahead at full tilt.

And then there are those amazing children.Tisha Campbell is a nonstop explosion of energy as the title character, and her talented entourage keeps pace with her from first song to last. "Really Rosie" is genuine family show -- my children enjoyed it even more than I did -- and it's genuinely well-produced too, in all respects.

It's more a novelty than a new musical classic, but it's a very novel novelty , and how often do those come along? My favorite moments involved the bookish Johnny, who ckecks the Readers Digest and Encyclopedia for information on "Sibling robbery" (how to get rid of your big brother) -- and a little girl who dreams of starring in her own movie, called "Cathy Grossman Meet Dracula."

Other spectators may prefer different scenes. But just about everyone should find "Really Rosie" a show to really enjoy.