"There, but by the grace of god, go I." shrug
It's all too easy to say "A happy meal cost £3, that's £15 a week!" But a parent might not have £15. But they have got £3.
"Food deserts" are a thing. Especially in heavily urbanised areas. There are expensive restaurants, niche/gourmet food shops, and fast food places, but no Asda, Lidl or Aldi. The best they can hope for might be a Tesco Metro, where prices are inflated, and ranges are small, to cater to the commuter grabbing a lunch, or a snack for the train, not for a struggling family with 2.4 kids.
Plus, things like education and culture come into it. Parents might not have a lot of knowledge as to what constitutes a healthy diet for a child. It was probably never modelled to them. And McDonalds is perhaps seen as a real treat in terms of dining out.
For those in deprived situations, us lucky Mumsnet posters might as well be from another planet. We speak the same language, and maybe live very close to each other, but our life experiences, and chances are a million miles away from each, to be unrecognisable.
We shouldn't be judging, we should be adding some extra items to the food bank donation points in our own supermarkets.