I'm a bit late to this thread but when I was young, the majority of children just were not overweight. If anything, the vast majority of children at my primary school were underweight.
I remember what kids used to eat back in the early seventies. Breakfast would be porridge or a boiled/poached egg and a slice of toast (no-one really ate cereal back then). School lunches would be an ice-cream scoop of mash potato, a 2.5 inch square piece of egg and bacon pie and a spoonful of peas with "pudding" being a 2 inch square of sponge and custard. Dinner would be meat and two veg with some smattering of chips or another scoop of mash.
All in all, an eight year old might roughly consume somewhere in the region of 800-1000 calories a day.
Now consider that a slice of pizza can be nearly 300 calories. If a child has a slice of pizza and chips for school lunch, you are looking at that meal alone possibly being about 450 calories. Add in a chocolate bar, and that goes up to 650 calories ... and that's just for lunch.
Back then, chocolate, crisps, sodas, and biscuits were all "special treats". Admittedly, I grew up in a very working class northern town, but even the middle class families I knew didn't buy cake, biscuits, chocolate or crisps on a weekly basis. If there was cake, it was because someone had baked it themselves for the end of the week. A bought cake was something you did for a special occasion. A sandwich for lunch was pretty much two slices of bread with a slice of ham or cheese, or a layer of shiphams beef or salmon paste. 
We just didn't eat processed food at all; no one really ate pasta either. When I was young, you used to see lines of women queuing up outside at the butchers and greengrocers every day. People ate plain meat or fish and veg with a bit of potato for their tea ... and that was pretty much it. If you were hard-up, you ate cheaper meat cuts (a pie made with stewing beef and offal, say, or some cheap mince fried with onions), sausages or eggs with your veg and potatoes, or maybe you'd have a couple of kippers. My grandfather used to eat tripe with vinegar if he was hungry.
When you consider it, these people were consuming way fewer calories than we do today. The only overweight people I remember in my childhood tended to be wealthy industrialists, landlords, or post-menopausal women that were at home and ate too many vanilla slices from the bakery.