Children, you are about to see what your mommies and daddies mean when they talk about the good old nights of live drama on television NBC is about to bring back in a limited but potentially significant way live national theater with "The Oldest Living Graduate," a comic drama by the late Preston Jones. It will be broadcast from Dallas tonight at 9 on Channel 4, just as live as it can be.

"People may see boom shadows, a camera may fall apart on the air, an actor may forget a line, and the producer may be throwing up in the control room," says David W. Rintels, the executive producer. "But that's the fun of it. That's the challenge of it. We're coming on nekkid and that is exciting, and if it transmits to the people at home, then it's worth all the tension that goes into it."

Many people in television think the public doesn't know the difference between live, tape and film. Great liberties are taken with the word "live"; concert albums are touted as having been "recorded live," an upcoming PBS "live from the Met" opera was actually taped in February, and the movie version of Gilda Radner's Broadway revue is titled "Gilda Live," which is certainly preferable to "Gilda Dead" but is still essentially meaningless.

"Live" means something is happening as you see it, and those who can remember live TV of the '50s know there was nothing quite so adventurous and electrifying as sharing the sense of immediacy and discovery that live dramas brought with them.

"The actors are thrilled about it," says Rintels from Hollywood. "They're leaving for Texas to do a play, a real play, not something that will be stitched together later on film or tape."

Rentels has signed with NBC to produce two live plays this year, between two and four in 1981, "and after that we'll see what happens." Naturally, given the flat and arid terrain of modern prime-time TV, one may be suspicious that a prestigious project like "NBC's Live Theater" is more of a pulbic-relations gimmick than a real attempt to upgrade the quality of the airwaves.

"In terms of [NBC president] Fred Silverman, I'm absolutely convinced it's not a gimmick to him," Rintels says. "If this is a gimmick, then so is 'Live From Studio 8-H,' and the Larry Gelbart 'United States' series and 'Project Peacock' [a new series of children's specials]. If they're gimmicks, they are very, very expensive ones.

"I'm sure it's not just public relations. I think Fred wants to see the network improve. It's a competitive situation, of course, but he's willing to do it the right way, and I give him full credit for it. My complaint with TV has always been that they don't hire the best people and then turn them loose. Now there seems to be a greater tendency to do that."

Rintels and his actors -- including Henry Fonda as the crusty, formidable codger of the play's title -- had three weeks to rehearse in Los Angeles before flying to Dallas for another week's rehearsal at Southern Methodist University, where the play will originate.

"We could have done it safer and cheaper, on tape or on film or something, but I think we've done it the best way," Rintel says. The single night's broadcast will cost about the same as a two-hour TV movie, somewhere beyond $1 million.

While "nobody has ever mentioned ratings" to Rintels in connection with this project, he naturally hopes that the play will attract a big audience. It ought to, Preston Jones was a populist playwright whom pettily provincial New York drama critics tended to ignore but whose works found great popular success throughout the country.

When Jones died last year after surgery for ulcers in Dallas, what could have been one of the major playwriting careers of this century came to an end. "Graduate" will be broadcast on what would have been his 44th birthday. Jones wanted to return the concerns of the stage to character and humanism and a slyly skeptical kind of hope. His "Texas Trilogy," of which "Graduate" is one-third, is destined to be performed for years to come in regional theaters throughout the country.

It is the American regional theater -- not a currently stagnant Broadway -- that the NBC Live Theater project is meant to celebrate. At the very least, "Graduate" will probably draw an audience of 10 million living-room playgoers tonight, more than have ever seen all of Jones' plays in theaters.

Encouragingly, too, if the broadcast is even a borderline success, it will be good, unquestionably good, for television. "I think television can create an appetite for its own shows, good or bad," says Rintels, who's been associated with many of the good ones. 'Gilligan's Island' begets another 'Gilligan's Island,' but hopefully what we're doing will beget other things like it.

"If it doesn't work, it will mean a setback for the next guy who wants to do something a little different, but I'm going to give it my best damn shot and -- let's hope."