Why don't parents take a stand against rise of childhood diabetes and obesity ?

Good question: There are several reasons that this is the case. Fast foods are cheap and we're in an economic crisis. Lack of proper education about healthy foods and the dangers of obesity is a major factor in most of our population. Cultural differences affect how many people look at obesity. Many don't see obesity as a problem at all. Food additives, the removal of fiber to preserve food shelf life...Etc.

Some do: Your question has a very complicated answer because childhood obesity is a national problem involving our western culture, economics, politics, the food industry, schools, home life, etc. Many parents do take a stand, but the problem is pervasive. Researchers, activists, politicians (mrs. Obama), doctors, etc, are on the case, but it will take a lot of resources and education for a big change.