I don't think it's down to money either. There's undoubt a group who are fixated on convenience foods, and the poorer of that group will buy chips and nuggets instead of ready prepared salad and a cooked chicken. But I don't think that's the full story. When I was skint, then like most of the skint people I know, you do a lot of cooking from scratch and bulking up meals with potatoes, veg, pasta etc, that leave you full till the next meal. Not pizzas with chocolate bars and crisps inbetween meals. When dd was little I was often too broke to buy much fruit, salad, fish, good meat etc. So I'd buy enough for dd and do without myself, cos that's what you do as a parent. I just got my calorific fill from cheaper food. At no point did it cross my mind to just buy a multipack of crisps to share instead of fresh food.
Plus people who are truly in poverty don't tend to drive everywhere, they walk a lot.
I know parents who are very overweight themselves who have brought up their kids to be healthy, so I don't buy that as the full explanation.
The worst junk food eaters I know, who've raised dc the same are just lazy/ deluded. They aren't skint, or suffering from mh issues, or struggling with disability, or necessarily the biggest people I know. More 'I'm perfectly fine on my diet of shite, with my curvy female figure, therefore my kids are too. I never liked sport either and it did me no harm. Growing kids need a layer of fat for illness anyway'.