JUMPING ROPE. Simple, you say. Simple, said the children watching the Kangaroo Kids warm up for a show at Prince Georges Plaza.

"He's not pickin' his feet up," observed a girl of the lone boy in the troupe, as he tripped. She allowed as how she was pretty good at jumping rope herself, especially double-Dutch.

The self-proclaimed experts in the crowd were promised a chance to prove themselves after the Kangaroo Kids did their routine, and the show began.

Wearing sneakers and red, white and blue uniforms with their names on the back, Howard County's Kangaroo Kids took up their sturdy "Kanga-Ropes."

The kids clicked their heels, hopped, kicked and twisted, never missing a beat. They swished the ropes left, right, like matador's capes. They criss-crossed the ropes and sometimes jumped two to a rope. One daredevil even jumped rope on a pogo stick.

They call themselves a precision jump-rope team. As two of their coaches, Jim McCleary and Caroline Stockham, looked on, the youngsters skipped their way through rhythmic aerobic routines called Jump choreographed to rock music and punctuated by the steady tap of the ropes smacking the mall floor. They used specially made Kanga- Ropes covered with colorful plastic tubing for durability.

Children in the audience seemed impressed. They clapped in time with the music and could hardly wait for the spectators' competition. Volunteers, divided into age groups, were challenged to jump for only one minute: The most turns of the rope won the contest.

Young children were encouraged to keep jumping even if they tripped. But McCleary told older participants they had to retire if they "messed up." The girl who preferred double-Dutch was eliminated, but her sister won the early-elementary category.

Though the Kangaroo Kids lent their ropes to the volunteers for the contest, enthusiastic parents bought the club's entire supply of Kanga-Ropes ($4 each). The proceeds help pay for the club's travel.

The overall winner was one of five adults who volunteered to jump. All the adults completed the minute without tripping. After jumping barefoot for 170 turns of the rope, Darlene Boston admitted, "I just wanted to see if I still had it in me." But, good as her performance was, the record for the Kangaroo Kids is an awe-inspiring 217 beats per minute.

Any healthy child can do most of this stuff. The key word is "healthy." Though it seems that all Americans are fitness buffs, statistics indicate otherwise. The President's Council on Physical Fitness says that one out of six children is physically underdeveloped, that half of the nation's school-age youth are overweight and 33 percent obese, and that fitness scores of children have not improved over the past 15 years.

Some critics blame television, home computers and video games. Others note that while the proliferation of team sports is encouraging, most team sports don't provide the continuous aerobic exercise that even a young body's cardiovascular system needs.

JumpAerobics enthusiasts say that's where jumping rope comes in. For example, "Total Motion," one particular two-minute routine by the Kangaroo Kids, is equivalent to running a mile, says McCleary. And rope jumping can be done indoors and out, requires only one small piece of equipment and doesn't oblige you to find a partner or a team.

In truth, simple rope jumping isn't difficult: that's actually the point. It's stamina and care that count -- and it's stamina that adults (and many children) lack, and care that the overtired or overconfident lack. Among four teenagers who participated in the spectators' contest at Prince Georges Plaza, all three overconfident high school boys jumped rapidly for half a minute, then lost their stamina or rhythm and tripped up. But the lone girl jumped steadily and carefully and emerged the winner.

JumpAerobics is the brainchild of Don Disney, formerly a physical education teacher at Atholton Elementary School in Columbia. He devised a pattern of 21 aerobic jump-rope movements for his students that combined balance, strength, agility, flexibility, power, speed, coordination, weight control and cardiorespiratory endurance.

But he wanted the youngsters to have fun, too, so he formed an after-school club, the Kangaroo Kids. Eventually the kids perfected their routines and took their show on the road. Disney became physical education coordinator for Howard County public schools and introduced his JumpAerobics to other schools there, leaving the Kangaroo Kids to parents and teachers who are trained to coach and who schedule the youngsters' appearances. This year the Kangaroo Kids number 97 and are divided into five squads ranging from kindergarteners to teenagers.

Traveling teams of Kangaroo Kids appeared recently at a convention of physical education teachers in Atlanta and at the halftime show of the Atlanta Hawks basketball game; at the Old Post Office pavilion and on the Mall; at a variety of schools and before community meetings, PTAs, scouts and clubs.

Several Maryland school systems also have jump rope teams (not all of them built around JumpAerobics) including Anne Arundel, Harford and Baltimore counties. They, and other teams, will be invited to the first Kangaroo Kids' Jump Rope Festival on Saturday, May 4, at Hammond High School in Columbia. Spectators are invited to watch the demonstrations.

WHERE TO SEE THEM The Kangaroo Kids will appear Saturday at 1 at Columbia Mall in Maryland; Saturday, April 20, at three Village Center locations in Columbia (10:10 at Owen Brown; 11:40 at Oakland Mills; 2:40 at Harper's Choice); Saturday, May 4, at the Kangaroo Kids' Jump Rope Festival, 9 to 3, at Hammond High School, 8800 Route 32 in Columbia; and June 1 at Harborplace in Baltimore. Scheduling coordinator is Jean Hodges, 301/490-1777.

For more information about JumpAerobics, adult classes, workshops, instructor training or Kanga-Ropes, contact Sam Andelman at Heartbeat Enterprises, 787 Oella Avenue, Ellicott City MD 21043. Call 301/461-5867.