It's not laziness, or television, or video games—it's what their goddamn parents are feeding them. Via ScienceDaily:

A new report from the EarlyBird Diabetes Study suggests that physical activity has little if any role to play in the obesity epidemic among children...

A review published in 2009 of all trials using physical activity to reduce childhood obesity showed weight loss amounting to just 90g (3oz) over three years...

It is well known that less active children are fatter, but that does not mean—as most people assume it does—that inactivity leads to fatness. It could equally well be the other way round: that obesity leads to inactivity...

The implications are profound for public health policy, because the physical activity of children (crucial to their fitness and well-being) may never improve unless the burgeoning levels of childhood obesity are first checked. If this cannot be achieved through physical activity, the focus has to be on what—and how much—children consume.

A UK newspaper adds:

The paper, published in the Archives of Disease in Childhood, suggests that overweight children may think about their body negatively, shying away from sports and exercise as a result.

This image of the overweight children who don't want to play—because they're embarrassed, because they're scared—is so, so sad. It's the parents who are lazy. And, arguably, cruel.