Picture a man in a mummy suit making a horror movie on location in a swamp. He gets a call that his wife has given birth to their first child in a nearby town and, not wanting to take the time to remove his costume and makeup, borrows a car and heads for the hospital. Then he runs out of gas. In a mummy costume. In a swamp.

If the possibilities sound at least faintly amusing, the results border on uproarious Sunday night (at 8 on Channel 4) when the plot, and the mummy, begin unraveling on Steven Spielberg's "Amazing Stories," the much-heralded NBC anthology series that, for a change, hits a beguiling and inviting stride. "Mummy, Daddy," written by Earl Pomerantz and directed by William Dear, demonstrates again that the best hope for the series is in lighter, sillier, whimsical tales rather than mining Spielberg's excessively ample sentimental streak.

"Mummy, Daddy" marks a change in policy for the production and the network, which up to this point had declined to screen episodes of the series in advance, fearing tattletale critics would give away plot surprises, or something. The surprises so far in the series have been predominantly gloomy ones. A change of heart by Spielberg and company regarding advance screenings may have something to do with the fact that ratings for the series so far have been unspectacular, which tends to make the series' lavish production budget look reckless. Also, there's the imminent arrival of the November Nielsen "sweeps," when ratings count more than at other times, to take into account.

Procedural matters aside, it is a pleasure to report in advance that "Mummy, Daddy" is a zingy screwball comedy, light and nimble and not plush-ponderous, like other installments of the series. Next week's more serious episode, "The Mission," directed by Spielberg, falls far short of its aspirations in the awe-striking, spine-tingling department, whereas the previously most successful "Amazing" was a funny comedy about an obnoxious teen-ager who was magnetized by a passing meteor. Message to Spielberg: Funnyness is preferable to amazingness.

We don't want to give too much about "Mummy, Daddy" away now, but it should be safe to reveal that Bronson Pinchot, of "Beverly Hills Cop," appears rather briefly as the director of the horror movie, a role that might be gently spoofing Spielberg himself -- or is that allowed? "This is a crucial scene!" he exclaims auteuristically at one point. "It's a magic moment! Where's my fog?" Later, phoning the hospital in search of his star, he is hung up upon when he says, "Hello. We're looking for a mummy that answers to the name of Harold."

In the course of the movie-mummy's odyssey, he encounters a real mummy, as well as a cousin of the famous blind hermit from "Bride of Frankenstein." This one, finding a mummy at his front door, politely offers, "Let me take your wrap." Certainly nothing else on television Sunday night will be dangling more pure fun in front of a viewer's eyes than this endearingly goofy half-hour. 'Children of the Night'

Network programmers once relied on shaggy dogs to boost the ratings of domestic sitcoms. Times have changed, and now they rely on teen-age hookers to boost the ratings of TV movies. "Children of the Night," the CBS Saturday Night Movie at 9 on Channel 9, drags still more fashionably dissipated tots across the screen, all in the simple shabby interest of venal sensationalism.

Ostensibly based on the true story of a PhD candidate who wanted to use adolescent prostitutes as the subject of her thesis, and then became a kind of den mother to some of them, the film doesn't get deep enough even to scratch the surface of a social problem, much less seriously investigate it. Instead, the conflicts are drawn in strict black and white and the melodrama tinged a sleazy purple.

Kathleen Quinlan plays the heroine, and Lar Park-Lincoln (who is not a hotel but an actress) plays the most troublesome of the delinquents. They have coarse and corny shouting matches, but you don't give a hoot who wins. Mario Van Peebles brings a certain brio to the role of the chief pimp, but his life style is made to seem so luxurious and carefree that impressionable young viewers might almost consider this a recruiting film.

"I don't turn anybody into hookers; their parents do that," the pimp tells the heroine. Later he says of a murdered teen-age prostitute, "The family that threw her away put her out there." Parents are not the only scapegoats in the simplistic scenario, however; men, we are reminded, are such beasts. Thus two thuggish vice cops are depicted as slavering sexists who say things to the do-gooding student like, "Don't you have some laundry to do or something?"

The preposterously lurid script by Vicki Patik and Robert Guenette may be more irritating than Quinlan's cold, erratic performance (she could give fidget lessons to Rosanna Arquette), but it's pretty much a tossup. "Hell is for children," according to the film's title song, but there's probably a special place in it for those who make pandering TV movies like this.