Ah, for the days of exercise innocence, when being a fit kid meant doing your best to dodge the dodge ball, to grasp the finer points of red light/green light, and to refrain from snorting when the gym teacher plugged in a peppy yet perplexing workout record that encouraged you to ''give that chicken fat back to the chicken and don't be chicken again.''
Judging from the current group of exercise videos for kids, those days are gone forever. Fun, fun, and more fun may be their stated theme, but the workouts are often challenging, and accompanied by plenty of talk about resting pulse rates, aerobic endurance, and, of course, the burn. There is a noticeable absence of poultry music.
The serious tone may be a reaction to recurring warnings that American children are in the midst of a fitness crisis. Recent studies by the President's Council on Physical Fitness and the Chrysler-AAU Physical Fitness Program have concluded that today's kids are less fit and often fatter than the children of a decade or so ago.
In hopes of reversing that trend, videos offer a mix of fun and exercise. The best include warm-ups, abdominal work, upper-body exercises, and aerobic routines. Careful attention is paid to proper form, and unrecommended moves, such as full sit-ups and bouncing stretches, are avoided.
Genuine enthusiasm for an exercise tape and its host (whether teen TV star Alyssa Milano or Mickey Mouse) can be one gauge of a tape's worth. ''The goal is to encourage children to adopt a more active lifestyle,'' says Dr. Russell Pate, chairman of the department of exercise science at the University of South Carolina. ''If they like a tape and like to do it, then it can be beneficial. If they hate it, though, and it's forced on them, it's not clear that it would be productive in the long run.'' Dr. Paul Dyment, chief of pediatrics at the Maine Medical Center, puts it more directly: ''Being forced to do exercise he or she hates will usually turn a person off to fitness forever.''
It helps if parents like a video, too. ''If we know anything about behavior, and health behavior specifically, it's that the parents' actions are a strong predictor of the child's,'' Dr. Pate says. Exercise experts are unanimously unenthusiastic about the use of exercise tapes as video babysitters; they contend that one of the most effective ways to help a child become physically fit is to make fitness a family affair. In the end, getting your money's worth from a tape and making it an effective tool for your child may depend on your ability to get into ''Goofy's Groove,'' or your willingness to ''get busy, get tough with Herschel.''
The bad news for video companies is that the success of adult exercise tapes hasn't been repeated with kids' offerings. The 77 top-selling fitness videos of all time do not include a single children's tape, says Leslie Grey, executive editor of Home Video Publisher.
''I think our problem is that many parents don't take the videos seriously; when they think kids' tapes, they think Nintendo and cartoons,'' says Paige Flink, director of new product development at HPG Home Video in Dallas.
Poor sales also may reflect a national shortsightedness about fitness at an early age. Dr. Arthur Weltman, director of the exercise physiology lab at the University of Virginia, says, ''We find that most Americans don't think about adopting a healthier lifestyle until they're middle-aged or older.''
Here is a survey of tapes designed to get people concentrating on fitness much earlier than that: