One might think that if television took even one more tiny step backward, it would fall off a cliff. Yet CBS goes retro on us again tonight with two new childish sitcoms that look tattered and threadbare on arrival: "Ace Crawford, Private Eye" at 8 and "Gun Shy" at 8:30, both on Channel 9.

Tim Conway, the second banana who never should have left the bunch, stars as "Ace Crawford," and the program, a timid spoof of detective shows of some other era, is a "Conway Enterprises Production." It does have fleeting, welcome flashes of antic silliness, as when Ace arms himself for an undercover caper with a child's walky-talky and a pair of quack-quack scissors.

Since sick comedy is now de rigueur in prime time, even at the 8 o'clock hour, jokes are made at the expense of a blind piano player (Bill Henderson) and a midget, played by the ever redoutable Billy Barty. A funny scene in which Conway invades Barty's tiny apartment and wreaks havoc by simply moving about is, however, much too reminiscent of the one hilarious sequence of "The Marx Brothers at the Circus," in which Groucho and Chico try to bum an incriminating cigar off the sideshow's Mr. Tiny and demolish his residence in the process.

Arnie Kogen wrote the pilot; Michael Preece directed it with no apparent aptitude for staging physical comedy. Joe Regalbuto, once one of "The Associates" on ABC, landed the hapless role of Conway's moronic flunky. He also narrates the show in mock-tough style. "Some people say" this and "some people say" that, he keeps saying, "and some people don't care." Unfortunately, Conway appears to be one of them, and whoever watches this show will probably join him in that apathy. Gun Shy

The prospect of a new half-hour comedy series from Walt Disney Productions sounds about as tantalizing as watching Ivory Soap float. But "Gun Shy" is from the new, allegedly hipped-up Disney factory, so jokes on the premiere involve death, suicide, lice, gambling, cheating and things that smell bad. There's even a drug joke; two sugar cubes laced with home-brewed speed, and meant for a horse, are instead ingested by a cowpoke, who then freaks out.

Yes, it's a long way from "Steamboat Willie." Too long a way.

If last year's moderately beguiling Western spoof "Best of the West," on ABC, didn't make it, how do CBS and Disney expect "Gun Shy" to fly? It barely flaps its wings. In the show, Barry Van Dyke plays a gambler with two cute young charges following him around, Bridgette Andersen and Keith Mitchell. According to production notes, he won the kids in a poker game.

Tim Thomerson (who also appeared on "The Associates," as it happens) and Geoffrey Lewis (Clint Eastwood's buddy in the "Any Which Way" pictures) could be funny in their continuing roles as a pair of stumble-bumble saddle tramps, the ones who cook up the speedballs, but they don't get enough latitude from director Peter Baldwin or enough Funny Stuff from writers Eric Cohen and Farnsworth Gallagher.

"Gun Shy" is idea-shy. It can race "Ace Crawford" to Boot Hill.