It's the heart of the summer, a time when a kid's thoughts may well turn to watching television — lots of television. But one television network is telling kids to turn off their sets, put aside their remotes and go outside and play.

"Our whole mantra is about a balanced lifestyle," said Marva Smalls, an executive vice president with the Nickelodeon cable channel, whose summer promotion this year is a series of specials called "Let's Just Play Go Healthy Challenge."

"Part of a balanced lifestyle," Smalls added, "is not to spend all of your waking hours watching TV."

Not that kids aren't encouraged to keep tuning in when they're not playing. One time the network wants them watching is Sunday at 8:30 p.m. (EDT/PDT) when the "Lets Just Play Go Healthy Challenge" kicks off its third and final month with a special program.

The segment, being produced in cooperation with the American Heart Association and the William J. Clinton Foundation, looks at the progress four youngsters from around the country have made in helping themselves and others improve such things as eating and exercise habits.

In some cases the improvements have been fairly astounding.

A 14-year-old from Alabama named Wes, who got involved in the program as one of Nickelodeon's four on-air challengers, has lost nearly 30 pounds as he and his family have switched from junk food to a healthier diet. Another, Chris from San Francisco, said he has gotten his entire school involved in changing its cafeteria menu, while still another, Bianca from Sleepy Hollow, N.Y., said she has been awakening relatives to the joys of gym workouts.

"It's going pretty good, better than I expected," she said.

Meanwhile, the network says it has signed up as many as 100,000 kids to take the challenge at home while watching the progress of the four on TV.

"Our hope for the outcome is we will have great traction in this movement, in this journey, to create the healthiest generation of kids among our audience," Smalls said.