How hard is it to adapt a children's book for the stage? Very. Just take a gander at the current Kennedy Center-commissioned "Lilly's Purple Plastic Purse," playing through Dec. 28 in the Theater Lab. Based on Kevin Henkes' charming tale of an effervescent mouse-child named Lilly and her up-and-down relationship with a terrific teacher, it's a mess of a show, a jangly, overdone, noisy storm of verbiage and movement. And these are not rank amateurs at work; a host of talented theatrical minds, among them playwright Kevin Kling, director Jerry Manning, set designer Tony Cisek and actress Rhea Seehorn have totally lost sight of the sly wit and piquantly crafted lessons in the author's text and drawings.

Kling's first mistake is to stuff extraneous characters and situations from Henkes' previous books into one unwieldy enterprise. The focus should be on Lilly and Mr. Slinger, the creative and simpatico teacher, not on verbose, goofy pals and caricatured relatives. And the dialogue tries too hard to cater to the grown-ups in the audience, turning the children in the play into smug sitcom characters. The production team has attempted to jazz up an understated story with blaring musical punctuations, overblown sound effects and a huge, garishly hued set worlds apart from Henkes' fanciful renderings of his characters.

So, too, has Manning directed his actors--all of them adults--to mug, scream, gyrate and otherwise punch up the material.

"She is so irritating!" groused the 8-year-old boy at my side, referring to Seehorn's portrayal of the central character. Indeed, this usually insightful and comedically on-the-mark actress turns the irrepressible mouse Lilly into an absolute neurotic, a tortured nut case in need of Ritalin. Braying her lines, galumphing about like a deranged pony, Seehorn is sadly unfunny and misguided.

The rest of the cast, each holding a variety of roles, pretty much follows her lead. Only Brilane Bowman, playing a bully, a grandmother and assorted other parts, manages to triumph by virtue of her amazingly evocative facial expressions and overall stage presence.

Lilly's Purple Plastic Purse, by Kevin Kling, based on the book by Kevin Henkes. Costumes by Howard Vincent Kurtz; lighting, Ayun Fedorcha; sound, Tony Angelini; choreography, Christopher Marlowe Roche. With Scott Hampton Cooke, Samarra Mbenga, Christopher Marlowe Roche, Christopher Walker. Through Dec. 28 at the Kennedy Center Theater Lab. Call 202-467-4600.

CAPTION: Rhea Seehorn in "Lilly's Purple Plastic Purse."