I used to eat Saltines by the sleeve (4 sleeves in a box). That may not be as horrible as some of the things mentioned, but the quantity makes it worse. I probably craved the salt, and didn't have the sense to just eat some by itself.
Growing up on a farm, we ate better than most kids of the era (70s and 80s), but far from perfect. On the plus side, we had our own meat and most of our own vegetables and fruit, and my mom cooked everything from scratch. No mixes, and they still don't have a microwave. We ate a lot of eggs, and fried things in homemade lard. Home-grown potatoes were a daily staple. We only got soda on birthdays, and didn't get sugar-coated cereals, just stuff like Wheaties and Cheerios (upon which we were allowed one spoonful of sugar per bowl). If we went into town for something like a doctor's appointment, we might get to split a package of Neccos when we stopped for groceries. Sounds pretty good, at least compared to what most kids were eating. A much lower sugar load, and a lot of good animal fats and proteins.
But when it came to the things we didn't grow ourselves, we tended to buy cheap (like most rural folks I've known). So margarine instead of butter, corn oil for anything calling for a liquid oil, processed cheese spread instead of cheese, the cheapest sandwich bread on the shelf, etc. We ate plenty of homemade baked goods, made with white flour, white sugar, and margarine or corn oil, plus plenty of cheap bread and pasta. Pancakes every Sunday, with corn syrup or jam. We made jellies and jams that are a marvel of culinary engineering -- how do you get two cups of sugar into a half-pint of jam? Sweet pickles were another sugar bomb: 8 cups of sugar in 5 quarts of pickles.
So we ate a lot of sugar (at least it wasn't HFCS) and refined carbs and vegetable oils, but still probably not as much as the average kid, and it was balanced by a lot of good stuff. We were pretty healthy kids -- maybe a little chunky at times, but not bad -- until we moved out on our own and dived into the soda and junk food we'd been missing all those years, and plumped up fast.