It's idiotic to single the cause of childhood and adolescent obesity and health complications to the child and the parent as the implications of this problem are far broader than that. Firstly there are many, many different reasons for why a child may or may not be overweight in their development.
Among the causes, poverty is a large one, as children in poverty often do not have access to quality healthy produce and other nutritious food products. The result is children who grow up eating the cheapest of goods that usually are made with cheap flavoring methods and lots of substitutes. This means the majority of their diet will consist of empty calories and is rich in sugars, sodium, detrimental fats and other health deterrents. Some children live in "food deserts" where grocery stores are not local as the companies cannot find monetary incentive to place new establishments where population is sparse.
Another cause is the food industries strong grip on the media, and their use of implementing their products as regular items that can be consumed on a daily basis. Many things they sell are advertised to be healthy and are promoted to be either very filling and tasty, or physically stimulating. McDonalds and Wendy's are two prime examples. McDonalds not only pushes childhood health advertisements to associate itself with healthy practices, but also sells toys to further entice children to ask their parents to eat at their establishments. Further more they use brand recognition methods to implement good and positive feelings in their brand logos. This allows children from a young age to associate their brands with positive, familiar and nostalgic feelings as a result of early commercial exposure. Wendy's most notable use of one of the previously stated methods is their advertisement of their fresh and "healthy" salads. While regular salads of the same size usually only contain 100 to 200 calories at a maximum, assuming large portions of added material such as croutons and heaver meats are not added, a Wendy's "BBQ Ranch Chicken Salad" provides 580 calories, including 9 grams of trans fat, 1460 mg of sodium, 25 grams of sugar and 140 mg of cholesterol. The difference between a real salad and what is advertised is seemingly miniscule besides the amazing and seemingly costless difference it taste, while in fact the meal nearly triples the total caloric value and exponentially increases the sodium and sugar content.
Yet another reason is the fact that the human biology dictates that we were never fully prepared to take on the amount of dense starches, grains and meats that have been incorporated in the majority of human diets in most western countries. These types of foods are usually accompanied by high amounts of sugars and salts. The result of this is an addiction that can start in early childhood. Children can easily start a cycle of craving either of both of these "seasonings" which have only been a part of our diet for what is less than an instant in the course of our evolution. While it can be argued that children who are old enough to understand that they should not be eating certain foods, addictions are indeed a powerful affliction in any form they appear in including overlooked addictions in many of the products that are eaten regularly by most notably North Americans.
Childhood obesity, and even obesity in general is a socioeconomic problem that has happened because of several factors that seem to always coexist in perfect harmony. Generalizing on such a wide scale, assuming that children or parents of all backgrounds share a common blame for the condition of health any many developed countries is completely and utterly ignorant.