is decrease in physical activity causing obesity to children

That's a great question.

I believe that a decrese in physical activity is only part of the problem.  We know about cuts in physical education and recess and the increased amount of time kids spend on the computer and in front of the TV, but we also have to remember the diet and nutrition part of the equation.  If they're trying to be active, but still eating tons of junk (which is more readily available than, say, 50 years ago) then it doesn't do them any good.  So we want to encourage not only the kids, but the whole family and other places, like our schools, to serve healthy, whole foods and we all need to help teach kids why these are better choices and be the examples for them.