As Murray said to Ted on Mary Tyler Moore's last show, "When a donkey flies, you don't blame him for not staying up that long." So, although the TV networks have fallen far short of complete reform when it comes to children's programming, one can't begrudge them a few bows for the progress that has been made.

Of course, activist groups had to go at them with crowbars to get them moving, but the results are what count. There are still bad children's shows on the air as well as good ones; the irony is that ABC is leading the way in both directions at once. On one hand, the network's Saturday morning line-up includes probably the most idiotic fun-fests since first was plunked a magic twanger.

CBS may have "The Robonic Stooges" and NBC "Baggy Pants and the Nitwits," but ABC takes the cake and the cookies as well with its showpiece of inanity, "The Krofft Supershow," featuring the adventures of "Magic Mongo," an effete fat genie; "Bigfoot and Wonderboy" who run around the woods, and "Wonderbug," a talking car that wheezes, belches and chases bad guys.

But the same network has also pioneered and perfected special programming for the young that has been of admirably consistent high quality. Today, ABC's innovative "Afterschool Specials" begin a sixth season with "Hewitt's Just Different," the beautifully told story of how a young Little Leaguer becomes friends with the retarded boy who lives next door.

Even though it is sure to be interrupted with the usual commercials for sugar-coated sugar, "Hewitt's Just Different," at 5 p.m. on Channel 7, is absolutely exemplary children's programming; and it's painlessly exemplary besides, because the characters in it are not just cardboard models but people of depth and vulnerability.

As often in television, the success of one network's initiative can be gauged by taking a look at imitations by the other two. CBS and NBC have both followed ABC's lead in airing image-boosting, worthwhile children's fare on occasional weekday afternoons. On Tuesday, NBC presented another of its "Special Treat" dramas, and on Thursday, at 4 p.m. on Channel 9, CBS begins a new monthly series, called "The Winners," about kids who assert their abilities and overcome obstacles.

The first show, "I Can," stars Debbie Phillips as herself in a fictionalized version of her own life. Born with one leg drastically shorter than the other, she nevertheless determines not to be left behind, particularly in the sport of horseback riding. She resolves a number of conflicts and wins the big race in the closing moments of the half-hour film.

It's certainly positive viewing, but the program lacks the sophistication and polish of ABC's. Both show were produced on shoestrings, but the CBS show looks it and ABC's doesn't, partly because "Afterschool" executive producer Daniel Wilson has by now got these things down to a science as well as an art.

Though ABC has fewer affiliated stations than the other two networks, more ABC stations carry the "Afterschool Specials" than carry the NBC or CBS specials for kids. Of 195 ABC affiliates, 186 air "Afterschool." But 20 of the 200 CBS stations will not be carrying "The Winners" and 37 of NBC's 217 affiliates didn't bother with "Special Treat." The reason is that local stations can make more money in these time slots with a butchered old movie or rerun of "Bewitched." Local stations are very interested in making more money.

"Hewitt's Just Different" isn't only a morality play striking a blow against prejudice toward the retarded. Its particulars carry overtones of universality; what 12-year-old Willie Arthur learns from his retarded pal, Hewitt Calder, has to do with the whole subject of friendship, with resisting peer group pressures nd with facing crises of conscience bravely.

The show is not only superbly written by Jan Hartman and alertly directed by Larry Elikann, but again it includes a cast of almost frighteningly impressive young actors, most notably, Perry Lang as Hewitt and Moosie Drier as Willie. The "Afterschool Specials" are like an off-Broadway training ground for young performers. Kristy McNichol went on to the prime-time "Family" and Lance Kerwin will star in the forthcoming NBC series "James at 15," to name two of many alumni.

The ABC Afterschool unit could teach the folks who produce so-called adult programming a thing or two and in fact they have. ABC Entertainment chief Fred Silverman not a look at the script for a two-part After school Special planned for later this season and decided it was so good that he scooped it up for prime time, where it will air in January.

"Roll of Thunder Hear My Cry," from Mildred Taylor's Newbery Award-winning book about a black family clinging to a piece of land during the Depression, has grown from a children's show into a family show and made the leap from afternoon to evening.

Considering the overall quality of ABC's nightly programming, this may not precisely constitute a giant step up; but it shows that good ideas still stand a chance in the ruthless world of network television. Oh, it's no revolution - but when a donkey flies . . .