I think if this were me, I would pause the visits and then in the New Year try and address this again.
i would talk with my DS and his wife to suggest for the next visit we meet out somewhere and I pay for the meal for them all. Similarly, suggest I take just my grandchildren out for tea
Or you suggest you will take food (ready meal/home cooked) for you all to eat at home. Agree a plan prior.
If they are reticent, ask directly, addressing the cost issue, (‘but this is not going to cost you anything’). Be ready to address other reasons ‘it is a school night’ - ‘ok, would Saturday be better?’
Now that you have had a conversation with your son, it is easy to pick that up, ‘can you remember when I said that I can’t come to your house and manage without food or a drink, would it be better if….’
Their reactions to this, as you plan ahead, would give more information about why they are doing this. If they can’t agree to any of the above, I would really know they just don’t want to see me.
I am in a similar situation with my mum, though she does make a cup of tea and is elderly. We just plan ahead with her and accept that we will ‘go out somewhere’ to eat, with us paying. We are not left hungry, we are no longer embarrassed by her behaviour (and can justify it by her age).
Your family are incredibly rude and other than my mum this would be unacceptable in our family. My DS’s both cook, make drinks, serve snacks.
However, I were you, I would not want to lose contact over it without trying a little bit more.