"Day by Day" has the Nutrasweet smell of success. NBC's post-yuppie sitcom about a white-collar couple running a preschool day-care center in their home gets no less than three "preview" airings on the network this week, the first at 8:30 tonight on Channel 4.

NBC Entertainment President Brandon Tartikoff is giving the show unusually generous exposure because it was cocreated by Gary David Goldberg, who did NBC a little favor called "Family Ties" six years ago. Like that long-runner, "Day by Day" is the very definition of a mild amusement, palatably positive and sunny, and wholesome in a forgivably plastic sort of way.

Brian and Kate Harper are, respectively, a former stockbroker and an ex-lawyer who six months ago decided to abandon those musty old pin-striped careers and turn their home into a way station for wisecracking toddlers. Toddlers apparently must be able to wisecrack in order to gain admission.

Thus does smart little Molly (played by an actress named Thora), cutest of the bunch, criticize the song "Old MacDonald Had a Farm" on the groundsthat "farm" is not spelled "e-i-e-i-o." Clearly, she was brought up on "Sesame Street."

The Harpers have two children of their own, a baby daughter and a teen-age son. It's a sitcom law now: There has to be a teen-age son. A wisecracking teen-age son -- this one named Ross and played by the unobjectionable C.B. Barnes. Just 15, Ross has a hopeless crush on the Harpers' helper Kristin, a college student majoring in child psychology and minoring in "textile arts."

She is played by the angelically pretty Courtney Thorne-Smith. Ross' efforts to impress her are funny and even borderline poignant, but let us hope this show doesn't turn out to be all about him the way "Family Ties" degenerated into the saga of young Alex, as played by Michael J. Fox.

There's further cause to worry in that regard since the father is played by dull Doug Sheehan, a dead ringer for limp Alan Thicke of "Growing Pains," which is an imitation of "Family Ties" that airs on ABC (a tangled web, this sitcom genealogy) and is usually about the adventures of yet another teen-age son.

A 12-year-old son in a sitcom would, at this point, constitute a programming revolution.

Linda Kelsey, so long ago a star reporter on "Lou Grant," brings a nice, bright, Jill Eikenberry lilt to the part of Kate Harper. Also in the cast is Julia Louis-Dreyfus, barely noticed when a member of "Saturday Night Live," playing a friend of the family who still clings to the yuppish dreams they have forsaken. She cracks a mean quip.

The first show tells a tidy tale about an estranged father who contemplates showing up at his son's 5th-birthday party. Andy Borowitz, also an executive producer, wrote it and Will Mackenzie directed. They had a relatively large number of tiny tots to work with in the cast and, for that reason, your heart goes out to them.

And to all those who sat through the retakes. Those taping sessions must be murder! But all in a comparatively good cause; "Day by Day" goes step by step in the right direction.

'Perfect People' Lauren Hutton and Perry King are certainly not out of their depth when it comes to playing shallow characters. In "Perfect People," the ABC movie at 9 tonight on Channel 7, they are Barbara and Kenneth Caldwell, two middle-aged airbrains who decide they've gone all shlumpy and take radical steps to regain lost youths.

In a previous era, the story might have been a fantasy about folks who rub a magic lamp and become young again. But medical science is the genie now. Hair transplants, tummy tucks, liposuction, organic diets and daily pilgrimages to the fitness center help the pair become the folks they used to be.

Alas, they're still terminally boring, but that is not the message of the movie. The message of the movie is that if you and your mate are run-down old drudges, at least you don't have to worry about each other attracting rivals for affection. Greg Goodell's lazy and fuzzy script resorts to a stale, dumb jealousy plot instead of attempting even semitrenchant social comedy about America's mania for youthfulness.

King and Hutton are kind of fun in the makeup that ages them (they bring to mind Crispin Glover and Lea Thompson in "Back to the Future"). When they shed their synthetic flab and wrinkles, they aren't even kind of fun any more. And Hutton's hair looks just as terrible after as it does before, which tends to confuse the point. She has one nice scene where she gets giggly with a pal over a hot fudge sundae after taking a diet pill, but it's over all too soon.

The rest of the movie is over all too late.