It's beginning to look too much like Christmas. Once again we find ourselves menaced by impending holidays and the inevitable family-oriented network TV specials designed to break down any lingering resistance to the buying urge. At no other time of year does so much cheap whimsy pass as scintillating fantasy.

"Once Upon a Brothers Grimm," the CBS special on channel 9 at 8 o'clock tonight, isn't the cheapest whimsy ever, but neither is ti a particularly rich or original holiday treat. It speaks not so much to the child in us all as to the pushover - the little voice that says, "Oh well, at least it's whole-some," even as another little voice is saying, "You're bored and you know it."

The two-hour videotaped special, with new and not unpleasant songs bu Mitch Leigh and Sammy Cahn, drops Dean Jones and Paul Sand as Joseph and Wilhelm Grimm into a Mgical forest where not all the magic is unprecedented. A witch throws balls of fire as in 'The Wizard of Oz," and Jones, momentarily a swan, sings a flying song a la "Peter Pan." Resemblance to such classic chilhood entertainments unfortunately stops there.

It's no thrill, either, to renew old acquaintaces with worn-out characters like Rumpelstilskin and the cannibalistic Gingerbread Lady. "Grimm" doesn't do much with them other than trot them out for easy recognition; the program begins to resemble the kiddie act at an ice show, in which the mere appearance of a familliar figure is supposed to generate squeals of delight.

One thing the program does have going for it is a splashy use of color. Prime-time TV has become an essentially pallid wasteland; there are hardly any production-number variety shows left, and survivors like "Donny and Marie" tend to look more hideious than phantasmagorical. Filmed series, especially those producted at Universal Studios, suffer from a sickly complexion of sullen green.

At least "Grimm" eschews the predominant Tv cool colors, orange and Blue, for some roaring hot ones. There's still too much soothing blue, but, as it happens, blue is crucial to the chroma-key technology that allows characters in the show to float around, fly about, and romp on sets that are actually just one-dimensional drawings.

Jean Holloway's script is flat and Norman Campbell's direction poky. A sequence that lent itself naturally to an outburst of real ballet is given over instead to pat TV choreography. And in a clumsy lapse of taste, the only substantial role given a black actor on the program is that of the wolf who tries to seduce Red Riding Hood.

The only thing worse than cheap whimsy is nasty fantasy. George Burns Special

"The George Burns One-Man Show" should have been called "The George Burns One-Joke Show," and the joke isn't one of your more appetizing zingers. All the guest stars are on hand, we are told, in case Burns, 81, collapses in the course of doing the program.

Crack after crack on the CBS special, at 10 p.m. on Channel 9, makes reference to how old Burns, is, how sick he allegedly looks, and how feeble he's become. At the opening, after a nurse has taken his pulse in the wings, the announcer says, "Now here he is to start the show, but we don't know if he'll be able to finish it."

Coming after months of show-biz deaths, including that of Burns' old friend Jack Benny, this kind of groveling for laughs seems especially misplaced and tactless - "sick" humor straight from Vegas. When we hear Burns say, yet again, "I'd go out with women my age, but there are not women my age," it's like being trapped with a Jewish comedian who has a penchant for anti-Semitic anecdotes.

On the one hand, the fact that Burns is still working has a clear affirmative aspect. But if all his jokes are going to depend on ridiculing old age, his professional longevity gets to be a questionable treasure.

Guests who come to his aid include Bob Hope, Ann-Margaret, the Captain and Tenille, and Gladys Knight and the Pips. Each does his or her thing, with only Knight - which is not to slight the Pips - coming across as fresh. John Denver also wants on for the obligatory plug of "Oh, God," the movie he made with Burns.

There are a few funny spots and one cute, genuine, spontaneous moment, when Burns is telling a long story involving Vincent Price and forgets Price's name. "What's-his-name, the great art collector," Burns says, and someone shouts out the name that escaped him. Burns smiles, puffs his cigar, and goes on with the story.