Where have all the heroes gone?

It happens all the time to Mr. Wizard and Sky King.

Don Herbert, who used to star in the popular children's television show ''Watch Mr. Wizard,'' will be walking along a street in southern California when someone will come up to him and say, ''I got interested in science because of your show. Today I'm working on the Saturn project at the Jet Propulsion Laboratory because of you.''

Or Kirby Grant, TV's ''Sky King,'' will board a commercial airliner and the pilot will recognize him. ''Ladies and gentlemen,'' the pilot will announce over the intercom, ''the reason I'm flying your plane today is because of a man sitting in the coach section.''

Mr. Wizard, Sky King, the Lone Ranger, Captain Midnight, Annie Oakley, Hopalong Cassidy, Roy Rogers and Dale Evans, Superman and Lois Lane - these were the heroes and heroines of the generation of Americans raised on children's television in the 1950s and early '60s. ''We decided to do things, to enter careers, because of these people,'' one fan recalls.

''I grew up in upstate New York and was the first kid on the block to have a TV - in 1951,'' he continues. ''By proxy, I was a very popular kid, and my friends and I watched all the Saturday morning shows. Sometimes we'd travel threE hours with our parents to Macy's in New York City to see Wild Bill Hickok in person. He was our hero. He was important to us.

''But who are kids going to go see 10 years from now? They're not going to try to meet Scooby-Doo because Scooby-Doo is an animated dog.''

Who are today's - and tomorrow's - heroes? Do children still need heroes?

These are some of the questions this fan, a former college teacher and television critic, is asking in his latest book, ''Saturday Morning T.V.'' (New York: Dell Books). An exhaustive look at ''Thirty Years of the Shows You Waited All Week to Watch,'' as the subtitle describes it, the book explores commercialism in television, violence in today's and yesterday's shows, today's cartoon shows, and the general spirit of live television.

When Gary Grossman was first approached by his publishers to do a nostalgic follow-up to his earlier book, ''Superman: Serial to Cereal,'' he told them no. ''I said, no, not nostalgia. I'm interested in doing a book on children's television, but only if I can be as critical as I want to be.''

In ''Saturday Morning T.V.'' Mr. Grossman is at his critical best. ''Some people may think they're getting a fun book with a lot of pictures,'' he says, ''but they're also getting a lot of anger - from people who shouldn't be unemployed, and who shouldn't be ignored by the medium they really love.''

Grossman spent more than a year researching his book and interviewing a variety of television personalities. ''I interviewed Lamb Chop (the hand puppet) , and Lamb Chop said things to me that Shari Lewis (Lamb Chop's creator) would not say. After all, Shari Lewis is working - she conducts symphony orchestras - but Lamb Chop is not working.

''Lamb Chop is the one who is highly critical of the networks today. Lamb Chop is the one who said that if you sent children's Saturday morning TV tHrough the mail today, it would be returned as junk mail.''

In his interviews with yesterday's stars, Grossman found one overriding, not surprising, criticism of today's children's programming: the lack of live actors. ''What they were talking about is the need for diversity,'' he explains. ''In the '50s and early '60s we had choices of 40 different shows to watch. Kids today don't have those choices.

''It really comes down to cost,'' he continues. ''You have to paiPyy- o to Roy Rogers, but you don't to Yogi Bear. In animation, the cartoon characters don't need health plans, they don't grow old, and they can be rerun ad infinitum. The scale for writing a cartoon show is much less than for writing a live-action show or a puppet show. And that, in essence, is why Saturday morning programming for children is almost exclusively cartoons.''

In addition to providing heroes and role models, Grossman says many of the early children's television shows taught some needed moral lessons. ''There were episodes that I remember to this day of 'The Rifleman,' because the relationship between the son and the father was so great. For example, the son was not allowed to have a gun. He knew that his father only used his gun when he was forced to, and then only defensively, and the son was punished if he took the gun. All through the show, the son wanted a gun, but his father kept saying, 'No. You learn to argue with the intellect, you learn to use your mind.' That was 4he theme of the show.

''There was violence on 'The Rifleman,' but it was restrained. And the only way violence is going to be restrained in society is if it's talked about. We need to talk about it constantly.''

If that sounds like wise advice from a grandfatherly type, Gary Grossman doesn't fit the image. He was only a few years older than his students when he began teaching several years ago at Boston's Emerson College, his alma mater and a leading communications school in the United States. From there, he branched into free-lance writing and soon landed a job as television critic with the Boston Herald American. He has recently begun a screenwriting career in California, where he also heads up his own television production company.

Grossman also worked for several years on the Boston-based ''Rex Trailgr Show.'' That experience, he says, and one other confirmed his love for television: One day, at age 4, a relative sneaked him onto the sound stage of th ''Howdy Doody'' show.''

I saw Buffalo Bob Smith, an actor, a nice person, and I saw puppets, and I saw strings, and I saw cameras, and I saw how they did it, and I saw something that was better than magic,'' he recalls. ''It's no accident that I'm writing about TV today because of that day.''

Grossman is quick to rate today's ''Sesame Street'' a ''wonderful'' show, but he nevertheless contends that it deals with young children in a ''somewhat idealistic, pristine world'' without addressing the kinds of issues that even young children can begin to think about. ''There are no heroes on 'Sesame Street ,' they've eliminated them, and as a result it's more comedy than social criticism,'' he notes. ''In the old days of the 'Kookla, Fran, and Ollie' show, Burr Tillstrom often didn't know what he was going to say before he opened his mouth on live television, but he was taking things right from the pages of the news.''

Although he doesn't think the Federal Communications Commission should be in the business of programming, Grossman does feel the FCC has an obligation to recognize that children are a special-interest group and have special needs. ''I think if the stations and networks were forced by the FCC to set aside time for live-host shows and puppet shows . . . the producers would be creative enough to come up with some outstanding new shows.''

What can be done to encourage better programming for children?

"I for one want to do more than just remember old times,'' Grossman replies. ''Unless I try to make things better, I'm being rather hypocritical, and I have a lot of former students who are going to 'get' me. . . . I do have a production company now. Unfortunately, I don't have any clout yet.

''But I think people can make a difference. While the networks don't listen to one or two people, they do listen to 100 people - as if they were 100,000 people. Letters to the networks do count, and they do have an impact.''