"Project Peacock," NBC's new series of occasional prime-time specials for children and their parents gets off to a very charming start tomorrow night at 7 on Channel 4 with "The Big Stuffed Dog," a sweet, whimsical bubble blown by "Peanuts" creator Charles M. Schulz.

The stuffed dog in question is a giant Snoopy, the prize possession of a child (Rossie Harris). It gets lost on an airline baggage conveyor belt -- as 90 percent of the world's valuables do -- and goes off on its own predominantly jolly adventure.

Part of this odyssey involves bringing companionship, a silent conversational partner, to an elderly woman (the invaluable Mildred Dunnock) whose grandson never comes to visit her. As almost never happens in TV fictions aimed at the young, this lovable character dies. Her few belongings are sold on the street, and the dog goes on to a new life as a carnival prize.

Others in the no-expense-spared cast include Noah Beery as Gramps, Mel Stewart as a building superintendent, Denise Nicholas as the nurse at a hospital where Snoopy's nose is sewed back on (as reassurance to kids waiting there for operations of their own) and Abe Vigoda, his face almost touching the ground, as the carnival barker.

Robert Fuest's direction could certainly have been sprightlier, but he does very well with the reunion between little boy and long-lost stuffed dog: It comes tumbling in slow motion out of a beautiful balloon flying over a lake.

Certainly this is a promising beginning for the "Project Peacock" endeavor and markedly superior to the cacophony and "pro-social" saccharine of the CBS "Sign-On" screecher seen earlier this week. Children's fare doesn't have to lecture or prattle to be worthwhile; it need only make contact with the remote region TV so religiously neglects, the human imagination. "Project Peacock; couldn't be more welcome. 'Kent State'

NBC's "Kent State" tries diplomatically to spread the guilt around. The three-hour film, about the 1970 tragedy in which nine college students were killed by National Guardsmen, depicts at least one student as a rabid troublemaker and one guardsman as a trigger-happy redneck.

It's hard to be dramatically potent, however, when one must also try to strike delicate balances. And so the film, at 8 tomorrow night on Channel 4, although intermittently moving and disturbing, finally sputters out on a note of numbing inconsequentiality.

This hardly means that an accusative, one-sided approach would have been preferable -- only that the project may have been doomed from the beginning by its own docudrama format and the commercial limitations of network television.

Some are depicted as true culprits. Former Ohio governor James A. Rhodes is portrayed (by fat Jerome Dempsey) as an idiot and an idiot who could have averted the tragedy had he followed the advice to pull the Guard out and phase the tumultuous campus back to normality.

This advice is given the governor while he is standing at a urinal -- perhaps a first, if a dubious threshhold for a TV movie.

Some performances are achingly heartfelt. Keith Gordon (of "Dressed to Kill") plays Jeff, a basically noncommittal youth destined to be the most famous victim, immortalized in a Pulitzer Prize-winning photograph of a shocked girl kneeling beside his body.

With a fellow student, he plays a game of "Where were you when" at breakfast (a game college students may still play, though they're probably running out of whens). After exhausting the subject of Woodstock, the assassination of President Kennedy and the Moonwalk, he complains, "Big stuff happens, and it's never where you are."

Some scenes and performances have a subtlety and temperance far superior to the retroactive hysterics one might have dreaded; writers Gerald Green and Richard Kramer composed a few very resonant images. One of the best scenes has two guardsmen trying to make college-level small talk with a couple of attractive coeds. One guardsman says that in her red blouse, one of the girls looks like "a rose."

Director James Goldstone has trouble, though, with the crowd scenes, which go on and on and are not very well photographed. The film becomes a grisly spectacle that attempts to duplicate the impact of newsreel footage but can't.

Several sources, including a book by James A. Michener, are cited as authenticating raw material for the screenplay, but then there's the virtually inevitable disclaimer at the end: "Some fictional characters have been introduced and some incidents modified for dramatic purposes."

About the only truly tasteless touches are a cameo appearance by a wire-service photographer who took the prizewinning picture and another by, NBC publicity announces, "a National Guardsman who was a member of the guard unit sent to Kent." How does that add anything to the film?

On a key night of the February ratings "sweeps," "Kent State" will air opposite the ripe and lusty "East of Eden" on ABC and the TV premiere of the Burt Reynolds "Hooper" on CBS. It is certain to attract the fewest number of viewers among the three -- not necessarily a situation to be bemoaned. 'Concrete Cowboys'

CBS, which loves to be thought of as "the Tiffany's of Networks," but which has long since plumbed the depths of the bargain basement, stoops still lower toinight with "Concrete Cowboys," a subhuman comedy premiering at 10 on Channel 9.

Kissin' kin to "The Dukes of Hazzard" and "Enos" and other programs that ought to give William Paley sleepless nights, "Cowboys" makes another pandering stab at mining a lode of Yahoo Chic. Its heroes are two dopey drifters who hoot and holler through pointless scrapes and feckless bellyflops.

Jerry Reed (of "Smokey and the Bandit") and Geoffrey Scott play the twosome, about as ingratiating as a pair of germs in a petri dish. The male characters in the show all have names like "T.G." "J.D.," "Buford" and "Jimmy Lee." The women -- well, they might as well all be named "Jeanine." "And CBS might as well stand for Cracker Broadcasting System.