A cute old white millionaire, the two cute black orphans he adopts, and a cute zaftig housekeeper plainly constitute a population explosion of adorability in "Diff'rent Strokes," the new comedy premiering at 8 tonight on Channel 4. Th program attempts an uncomfortable parlay of insult jokes and marzipanian sentimentality.

Conrad Bain, so winningly the conservative doctor-next-door on "Maude," plays Phillip Drummond, a Park Avenue potentate who relocates from Harley the two young sons of his first housekeeper after she dies. We are told in Ben Starr's script that Drummond does this out of love and devotion to the departed maid: yet he apparently never had enough interest to meet her sons while she was alive, and paid her so little that she lived in poverty.

Thus it takes a certain WASP-liberal, noblesse-oblige attitude to find his gesture very generous. The two kids do seem aware of the underlying ironies and respond to Drummond with suspicion and with wisercracks so formula-slick that the two of them could probably get work as gag writers for Bob Hope.

The true star of the show is 10-year old Gary Coleman as 8-year-old Arnold. Coleman, who underwent a kidney transplant, is a most unusual tot with a strikingly professional comic delivery. But the writers insist on making him a half-pint Don Rickles. When Arnold meets Drummond's daughter, a condescending sort who wears braces, he snaps. "Hi, metal mouth," and later says of her, "Better not mess with me, or I'll reach up and belt her in the knees."

Of Drummond's male secretary. Arnold observes, "He looks like the kind of guy who goes to the toilet a lot," and when the millionaire tells the kids he has a hot tub. Arnold, thinking he means a stolen tub, says, "If we help you fence it, we get half."

At Tandem Productions, a spokesman boasts that the show is refreshing because millionaire Drummond is totally "color blind," but it seems the writer is not, because too many of the lines depend on stereotypes of one fashion or another. He even puts into Arnold's mouth that tired old Fetchitism, "I think we died and went to heaven."

Charlotte Rae, once of TV's "Hot I Baltimore," is a high-comic asset as the maid, and 13-year-old Todd Bridges, as the other brother, convincingly projects the skepticism of a kid too familiar with the realities of a hard life. When this little household stops fighting intramurally and bands together against threats from the world outside, "Diff'rent Strokes" may evolve into the kind of show millions of diff'rent folks will enjoy.

Asked which program should be considered "the first Fred Silverman show" on NBC under his new regime, the president and chief executive officer cited this one. It does not make a fully satisfying or commendable curtain-raiser, but it deserves a chance to blossom.