The old adage "children should be seen and not heard" certainly does not apply to children's opera. But neither does the corollary - i.e., that it is best heard and not seen, obviously the case with conventional recordings.

Here are two cases in point, Oliver Knussen's and Maurice Sendak's translation to the operatic stage of the latter's popular children's books, "Where the Wild Things Are" and "Higglety Pigglety Pop!"In his liner note for the Arabesque recording of the first, the composer acknowledges that "the physical climaxes of the opera . . . are, like their original sources in the book, entirely nonverbal." Which means that, in addition to hearing the music that accompanies these scenes, such as the rising of the sea monster and the dance/orgy of Scene 6, one should be able to see the stage action, especially as it has been conceived for this Glyndebourne Festival Opera video.

In addition to the libretto, Sendak himself supplied the set and costume designs and they are everything one would expect, the wild creatures monstrously outsize, with fearsome teeth and horns, yet possessed of a lumbering softness and faces of Muppet-like mobility.

Operatically the premise is not unlike that of Ravel's "L'Enfant et les sortileges" ("The Child and the Fantasies"). Sent to bed without his supper, a small boy, Max, imagines a voyage to a fantastical island where he becomes king of the Wild Things, ultimately sending them to bed without their supper. Upon which he sails home to Mama and a bowl of hot soup. Musically, however, the debt is to Debussy and Mussorgsky (Max is in fact crowned to music from the Coronation Scene from "Boris Godunov") albeit in a more advanced 20th-century idiom interlaced with some infectiously jazzy wind solos.

Maybe even more than on the Arabesque recording, Knussen & Co. serve all this up with impressive conviction. Similarly I prefer soprano Karen Beardsley - remarkably believable as Max - to Arabesque's Rosemary Hardy. Certainly she projects better, even if the vocal lines are less important than the instrumental. Add to that the visual element, really essential here, and you have what seems to me the definitive representation of this 38-minute opera in any format.

As a bonus Pioneer has included, on Side 2, the Knussen-Sendak "Higglety Pigglety Pop!" which at 58 minutes I find a more variable prospect. (The two are available separately on videocassette from Home Vision, $29.95 apiece.)

For one thing not only is the libretto, about the adventures of Sendak's Sealyham terrier Jennie, considerably more diffuse, but Knussen's music is likewise darker and more unsettled, a fantasy score that borders on the surreal. In my experience children are seldom put off by modernism per se - indeed, they are probably more open to it than their elders. But I do believe they tend to be put off by obscurity, something Knussen's Mozart/Mussorgsky-by-way-of-Stravinsky does little to clarify.

I suspect they will like the tuneful "Chicken Little" song, however, (reminiscent of Feodor's chatter about the parakeet in "Boris") as well as the Fafner-ish Lion (sonorously voiced by Stephen Richardson). And if poor Jennie (Cynthia Buchan) too often seems caught in a world somewhere between Kafka and "Alice and Wonderland," the title number at the end - almost a classical send-up of "Hickory Dickory Dock" - is pleasing enough, even if it expires on a puzzlingly inconclusive note.

But then maybe your kids like the surreal.