The affordability of healthy food is another issue, but related for sure. However, I think that for the most part, to become obese actually is quite expensive, considering the amount of calories that are required, so it is unlikely a case of affordability of better options in many of the current cases at least.
However, the money argument isn't valid for much of the world where obesity is an issue and it isn't an issue around where I live, yet there are plenty of obese children.
When my wife was young, her parents kept getting called into nurses and doctors because they thought she wasn't being fed enough, yet that wasn't the case, she is just very small. However, none of the parents are getting reported for their children being overweight. Why the double standard?