This topic is so complicated that it's hard to know where to begin. Read Marion Nestle and Michael Pollan and Christiane Northrup and Eric Schlosser and many many others!
First, let's acknowledge the difference between a child being morbidly obese and being of a slightly heavier build than perhaps the average healthy child. I mean, there is a range of normal and we'd do very well in this country to accept the beauty of different body types. I'll add to all the other pp.s who commented on the photograph of the other pp's daughter: she looks perfect!
Let me remark, too, that I so often talk to or encounter interviews and such with women who have battled weight problems for so long and who went on to begin eating healthfully but who say things like, "Oh, when my kids are eating pizza, I now have the willpower to just have a salad!" Now, not that there's anything wrong with pizza now and then, but I often think how funny that women who clearly have had important epiphanies about their OWN relationship with food don't seem to make the connection to their kids and see the necessity of instill in their children healthy eating habits so that their KIDS might avoid the problems they themselves have faced all their lives. What can we do about that?
Now, the question of the relationship between poverty and obesity is a very tangly one. One pp mentioned that many healthy foods are actually CHEAPER than junk. As I understand it, that is true when considering the WEIGHT of each. But when it comes to calories, processed foods are cheaper. In other words, a pound of tomatoes IS CHEAPER than a pound of Cheetos. But 200 calories of tomato are MORE EXPENSIVE than 200 calories of Cheetos. This might seem like a trivial difference, but when you are truly poor and your children are truly hungry, you need to get as many calories as you can for your money. Cheetos (etc.) starts to look like a pretty viable option if want your kids to go to bed full instead of hungry. This gets into all sorts of questions of what the government can/should be doing to encourage or enable poor families to feed their children nutritious foods. It's a topic that's probably outside the scope of this thread.
I'd also like to mention that there is increasing evidence that not all calories are created equal. This is part of the issue of the so-called "French Paradox": that French people (and Italians and Greeks and many, many other traditional cultures) eat rich, fatty foods, but are thinner than their American counterparts (we, a people who are obsessed with weight and dieting). Many of you who know Weston Price know that in his research, he observed that wherever a modern, Western diet (full of refined sugars and processed foods) was introduced to a traditional one, rates of obesity, diabetes, heart disease, etc. soared. Another FASCINATING book that touches on this is Hungry Planet, which looks in a series of photographs and articles at a week's worth of food for people around the globe (read about the aborigine family in Melbourne, Australia, for instance). Or, to boil that all down to one sentence: you'd be better off eating spinach wilted in a pan of bacon and bacon fat than a twinkie! (Read: Fat: An Appreciation of a Misunderstood Ingredient, with Recipes by Jennifer McLagan.)
We have a pretty messed-up food culture here in America (read Michael Pollan's In Defense of Food: An Eater's Manifesto, a follow-up to the Omnivore's Dilemma), and this fact prevents me from outright accusing parents of morbidly obese children of a form of child abuse (although they certainly deserve some of the responsibility). Advertising to children, fast food, eating in cars, the ubiquity of processed food, a lack of viable food culture, misleading health claims coming from food industry, exploding portion sizes, and ignorance about what healthy food actually IS all conspire to make people fat and unhealthy.
Of course parents have a responsibility to feed their kids healthfully! But I would also argue that the government should not subsidize the corn and soy that appear in ALL our fatty, nutrition-free processed foods, nor should corporations be entitled to make fairly warrantless health claims about these processed junk foods, nor should schools have Coca-Cola vending machines and greasy processed foods for lunch...and the list goes on.
It's a complex and important problem, one that Alice Waters said was so vital that she couldn't vote for any candidate that didn't address it.
This is a subject that I am really passionate about, can you tell!