Fast food adds where there when I was a child, and rarely did you see a child that was obese. I don't think that is the problem.
I think it has a lot to do with our lifestyle now. Too much being on the computer(and I mean parents too), playing video games, watching tv, not enough good outside play time. Also, with the lives we live these days, both parents working, single parents, it becomes difficult for most to set out a healthy diet with the kids, much easier on the tired mom or dad to just pick up something at a fast food restaurant.
As a kid I only got fast food once in a blue moon, now days I hear of some kids having it 4 to 5 times a week.