With characteristic divination, Harlequin Dinner Theater has hit on a neglected musical charmer for its new attraction, "Once Upon a Mattress."

This is the witty little romp which introduced Carol Burnett in a Mary Rodgers score in 1959. It's a variation on an old fairy tale, "The Princess and the Pea," and is ideal for a family night out. Gentle enough for the young 'uns, smooth enough for their elders, this is satire in the dress of innocent make-believe.

Queen Aggravain does all she can to block any marriage for her slowwitted son, Prince Dauntless the Drab. A final contestant, Princess Winnifred the Woebegone, arrives dripping wet: "I swam the moat," she keeps boasting. King Sextimus the Silent, who can't speak, mimes about birds, bees and storks to Dauntless in "Man to Man Talk," innocently amusing.

Fred, as the Princess wants to be called, is as backward and honest as Dauntless, singing "Shy" and, a lovely spoof, "The Swamps of Home," to explain herself. Another engaging character arrives late, a caged nightingale named Ucella Rosignolla. The book-writing trio, Jay Thompson, Marshall Baker and Dean Fuller, did a neat job.

Harlequin's large singing cast is aptly costumed in colorful medieval dress, and Richard Wall's well-drilled band of nine sets the Rodgers score to assured tempos.

Barbara Walsh, who scored in "The Boys from Syracuse," has lively fun as Fred and Lee Anne Duncan is spirited as the wicked queen. Oran sandel brings a lovely voice to the minstrel. The romantic subplot is well handled by Megan McFarland and Tony Gilbert, with Roberta Powell as Rosignolla.

Jeff Farnsworth has handled the staging with his tongue at just the right saucy angle and there are pleasing dance contributions led by Brian Donnelly, Gerry McCarthy and Julie Beaudoin.

Performances are Tuesdays through Sundays; reservations at 340-8515.