Uncle Johnny Coons used to tell us TV kiddies of the '50s to move back from our TV screens during "Lunchtime Little Theater" so that, we assumed, there'd be less danger of going blind from watching those "Crusader Rabbit" cartoons he used to show.

Uncle Johnny never suggested, however, that we be so brazen as to get up and leave the room while he was on the air. That would've been video heresy. Now, however, a Los Angeles TV station is planning a children's program that will do just that.

It will even go so far as to suggest an occasional expedition prior to a commercial.

"Sunday Funday," which premieres on ABC-owned KABC-TV Sept. 11, will at regular intervals urge its 6-to 12-year-old viewers to tear themselves away from the primal tube and go on little educational missions. They will not be encouraged to give up television altogether, nor even really to cut down on their daily viewing, but they will be reminded that there is a world beyond Fonzi, Farrah and ring-around-the-collar.

The project is the pet of John Goldhammer, a former executive at Washington's WTOP and now KABC program director. Goldhammer says "Funday" will be a weekly live hour at 9:30 Sunday mornings designed to inspire kids to "Get away, get up, and move" from their fixed positions before that altar of commerce, the television set."I think we in television have gotten marvelous at children's programming," Goldhammer says, "but we've also tied the child more and more to the set, so that they do all their reality testing in front of the set rather than elsewhere. Television is a two-dimensional tool; it should be only one sphere of a child's reality testing experience, not the whole thing."

So, "Funday" will challenge kids to, say, go to the dictionary and look up an unfamiliar word flashed on the screen, then go to a telephone, dial the station, and try to be first with the correct definition.

An inducement will be the promise of prizes and awards. You wouldn't expect them to open a book just for the fun of it, would you?

Goldhammer says other kinds of informational scavenger hunts will be tried. Occasionally, he insists, the tease to find an answer will come right smack before a commercial break - meaning the kids could miss another chapter in the continuing saga of the Honeycomb Hide-Out, or miss hearing for the 10,000th time that Trix are for kids.

It's not hard to imagine the folks in charge of the Honeycomb Hide-Out or America's Trix-makers getting a little grumpy about their target audience leaving the room to look up "avaricious" just as their commercials come on. But Goldhammer isn't worried about a sponsor revolt. He says that if only 20 per cent of the children leave the room on missions, that would be a strong response, and also leave the advertiser with 80 per cent of the audience still spell-bound.

Kids will never be encouraged to leave the house while the show is on, nor to turn off the set. But "Funday" could be the first TV show in history to say the opposite of "Don't Go Away."

It's an intriguing concept, and one that other cities might copy if it catches on. Of course, the real prime time for kiddies is Saturday morning, not Sunday, and the show would be a bolder step if placed there, in the domain of the Grape Ape, Dyno-Mutt, Speed Buggy and Dr. Shrinker.

Certainly the idea shouldn't stop at children's shows, either. Adult programs could offer occasional inducements for leaving the set - on purpose, that is. There are already plenty of programs that virtually command you to clear out of the room, if not leave the house, if not take a Carribean cruise - but only because they're lousy.

Imagine Baretta turning to the camera now and then to say, "Folks this next scene is gonna be so darn violent that I think you might be happer if you split for a while."

Or Howard Cosell coming clean with, "Tonight, I am going tobe so extraordinarily obnoxious that only a seasoned masochist of the most imperturbible countenance would even consider enduring me in my entirely."

Or maybe just a simple announcement at the beginning of "Donnie and Marie," like, "Extensive viewing not recommended for people who want to hang on to what's left of their marbles."

Television has such possibilities.