I saw that post, and gosh, yes difficult. Of course nappies and suchlike should be provided, but for one day a week for one small child it would be better for the grandparents to buy things the child likes and keep them there and mum pay for them, I think the mum mentioned fish fingers, and driving nearly an hour to them, by that time they would have been half melted. A few bits like that don't take up much room do they. It needs to be made easier for both of them. I couldn't wade through all of it but did wonder if that set of grandparents had put pressure on them to have the child as the other grandparents had the child too, certainly it did not sound like an ideal solution. I have to declare an interest as when my children were small because of eczema they were on soya milk, and we would visit for the weekend and sometimes it was on the train. They would not buy a box of soya milk when they were at the supermarket , buying their cows milk, so always had to bring it. My mother saw no problem in popping a box in the trolley when we visited my parents also a longish trip. Or other things that they liked, such as juice, the other ones it was water or beer, so it was not the cost, as am sure that a bottle of beer cost more than juice, which we watered down, did. And obviously the only choice was water as toddlers are too small for beer, but it was the same when I was pregnant. If we didn't take extra food we'd sometimes be hungry, again, a pain on the train, or even in the car, as even if we'd been asked to arrive at teatime, and they knew what time we were coming, tea was never ready [eventually we got wise to that and ate en route]; when they visited we took account of their likes and dislikes and always made sure that there as plenty to eat as that's just me, I find it most hospitable if visitors are well fed with food that we know they like.
So sometimes there can be deeper issues, and the complaint about needing to bring food etc can just be a clash of cultures