To those who have been following the saga of the kid-vid world, the latest drama may sound like something out of an old vaudeville routine:

On stage, a man encounters a woman just back from a week in the Catskills. "How was the food?" he asks.

"Just terrible," she answers, and so little of it."

In the same spirit, the advocates of children's television have spent years decrying the quality of programming, and now they are lamenting the decline in quantity. If it's so terrible, why should we care that there's less and less of it?

Well, it turns out that there is a reason, in the past year of rapid kid-vid big change, it's the better programs and, above all, the promises that have been cut.

The star of the disappearing act is "Captain Kangaroo," the only daily network program for children. This old favorite was sliced from an hour to a half-hour, to make room for an expanded CBS Morning News. It now shows every sign of being canceled.

In several cities, programs like the highly acclaimed "The Great Space Coasters" and "Romper Room" have faded from five times a week to once a week.

What it adds up to is simply this: at this moment, over 50 percent of the nation's television stations have no, repeat no, kids' programming between 2 and 6 p.m. weekdays. There is also no, repeat no, commercial network programming regularly scheduled for kids on weekday afternoons.

Children's programming, which was tiptoeing into the daily world and planning a leap or two, has been pushed back to the kid-vid ghetto of the Saturday morning cartoon clones.

Even there, the backsliding signals are being beamed loud and clear: NBC, which added health and sports features last year, has subtracted them this year ABC, which promised proudly to subtract two minutes of ads from each kid hour, has already added back 30 seconds.

Something happened or, to be more precise, someone happened. Last May, a former rock-radio disc jockey named Mark Fowler was appointed head of the Federal Communications Commission. Fowler is only one of the assorted foxes to be put in charge of the regulatory roosts by President Reagan.

While the others call themselves deregulators, Fowler one-ups them by referring to himself as an "un-regulator."

Past commissioners have used their post to remind broadcasters of their responsibility to the smallest, most impressionable viewers. But Fowler has used his post to bawl out broadcasters for ever allowing themselves to be regulated.

In his maiden speech to the International Radio and Television Society last September, he set himself up as a kind of Jim Jones of the FCC. "As regulators," he said, "we must be ready to self-destruct.... I know our staff is ready to meet this challenge...."

Fowler then offered his own theory of un-regulation: "The commission should so far as possible defer to the broadcasters' judgment about how best to compete for viewers and listeners because this serves the viewers' interests."

Peggy Charren of Action for Children's Television refers to this as the Trickle-Down Theory of Communications. "What's good for CBS is good for the audience. What's good for Hanna-Barbera [the cartoonists] is good for migrant children."

Until now, the FCC has been the Damocles sword held over the head of broadcasters. Stations have had to fulfill certain public interest guidelines or risk their licenses. But not any more. The broadcasters got the essential message They could stop worrying about expanding children's programming.

When the marketplace prevails, kids TV generally fails. Kids don't buy enough beer, deodorant or soap powder. With insufficient headaches and hemorrhoids, they end up plugged into Saturday mornings or lumped into the pre-Christmas specials between toy commercials. As John Claster, the producer of "The Great Space Coaster" laments, "In fairness to stations, the children's area isn't the most lucrative. And the younger age group isn't very good at defending itself."

One thing has remained the same. Kids under 12 are still watching an average of 27 hours of TV a week, most of it adult fare, much of it inappropriate and indigestible.

In this vaudeville act, it may be terrible fare, but the portions are enormous.