Obesity in children is definitely a problem today. The U.S. Centers for Disease Control tells us (April 24, 2015),

The percentage of children aged 6–11 years in the United States who were obese increased from 7% in 1980 to nearly 18% in 2012. Similarly, the percentage of adolescents aged 12–19 years who were obese increased from 5% to nearly 21% over the same period.

Obese children may find it hard to lose the weight that compromises their health in adolescence and adulthood. And the relationship between sedentary lifestyles, which the Internet encourages, and obesity is common sense as well as research. But to understand the problem, we need to go back a few decades.
Former Boston Globe columnist Maggie Jackson asks us to consider a “little known academic study” from about 40 years past: Roger Hart’s Children's Experience of Place: 

Hart spent two years in a small New England town, following around children as they built forts in their backyards, fished at the local river, explored, bicycled, roamed and wandered. It seems amazing that his depictions of life not that long ago seem worlds away from the indoors-centric, cyber-dominant, car-oriented lives of our kids today.

I remember that lifestyle. I lived it in the late 1950s, passing the Yukon River, on my way home from school.
The World War II generation in Canada was raising one of the largest baby booms in post-war history and did not have time for helicopter parenting. They assumed their children would know enough not to jump into a swift, icy torrent, as they themselves would not have done in their own youth.
I cannot recall a child back then who was obese. As I tried to explain some years ago,

Children then tucked happily into greasy grilled cheese sandwiches, heaps of buttered potatoes drowned in rivers of gravy, and huge banana splits whenever we could get them—indulgences that would horrify today's calorie-conscious, hyper-health supermom.

But . . . in that pre-microwave era, cooking was labor intensive, so children ate mostly at home at mealtimes. Between home and school we were largely unsupervised—definitely a no-no today—and we rode bikes, swam, or ran for hours on end.

Fat? Most of us couldn't get fat if we tried.

Today, by contrast, the internet turns out kids whose best-exercised body parts are their index fingers. But what parent of an obese child is going to move to a remote area, give away the TV, the Xbox, and the microwave, buy the kid a rattletrap bike and helmet, and go back to peeling potatoes?

Today, I might add that a number of new tech devices like smartphones, iPads, and iPods mainly exercise fingers and eyes.
Fingers do not, incidentally, have muscles in the usual sense; they are controlled by tendons from the muscles closer to the brain. So one cannot even strengthen the “muscles” of one’s fingers by giving them exercise manipulating digital media.
Jackson, the author of Distracted: The Erosion of Attention and the Coming Dark Age, offers,

Today, I’ve heard it argued that the Net is kids’ backyard. This is a space for a thin kind of social connectivity, and for exploring worlds largely of adults’ imaginations. But the virtual isn’t a space for coming to grips with one’s own place in the physical world, or for exploring the planet earth.

Consider that natural spaces – even a walk in the park – diminish symptoms of ADHD and improve focus in children even without attention deficiencies. Consider that kids in an age of alarming obesity are spending just 30 minutes of unstructured time outdoors – a week! Consider that kids today are living under a kind of house arrest, unable to walk to school, play outdoors, explore their own communities.

The Internet woos kids away from physical activity in part because it is a live medium. What the child sees may actually be happening somewhere right now. But that makes the temptation to passive spectatorship much stronger.
As a kid nearly sixty years ago, stuck inside during a three-day snow- or rainstorm, I was reading the Encyclopedia or else back issues of Reader’s Digest. Not very alive, neither of them. And hardly an incentive to stay inside once the weather improved.
By contrast, the Internet is a complete, satisfying non-physical world. No surprise that so many children’s physiques today reveal that fact.
Of course, giving the child real challenges, as per the vid below, might help. With an assist from social media. But parents must then expect to be challenged to get in shape themselves, to model how it is done. Could that be the biggest challenge? ;)
 

Denyse O’Leary is a Canadian journalist, author, and blogger.