Why is child obesity more common in lower income families than in higher income households?

Cheap food: Many reasons: cheap food is one. Salmon is $11.99 a pound and most fast food places have a "dollar menu." exercise is one: higher ses families can afford sports/lessons/gym/etc. Knowledge is one: an understanding of the problem and the education to know that prevention is vip. There are many more, I am sure, but not much room here...

Lack of education: Many low income familes lack education as to what is healthy versus less healthy food. When asked about their diet, many low income families think that their diet is pretty healthy when that is not the truth.