Your grandchildren are 6 and 8, so they are old enough to understand that you do things differently in your house than their parents, so if you establish some ground rules the children will quite probably follow them without blinking.
The test of your diplomatic skills is negotiating this with your DS and DIL!
Your DIL sounds a nice person. She said, and I assume meant, that she didn't expect you to feed and entertain them.
In good time before their next visit, write or phone your DIL and say you would be happy if you (the adults in the family) could share some of the shopping, cooking etc. either by doing it together, or by taking turns. Now is also the time to start teaching your grandchildren to help in the kitchen. IMO no child should leave home at 18 or whenever unable to cook ordinary, nourishing and cheap food.
In your place I would insist, politely, but in a way that indicates that the matter is not open for negotiation, that everyone sits at the table while eating, and that children may "get down" when they have finished, but then they do not come back to the table. I would also ban snacks, they are unhealthy and ruin appetites for the meals, but I would be very willing to negotiate meal-times.
When I was a child, learning French at school, we were told that no French children ate the evening meal with their parents, but were served a meal at 5 or 6 p.m and were in bed before the grown-ups ate at 9 or 10. Worth considering, perhaps?
Also, insist that "at my time of life I need an afternoon rest" so for two hours every afternoon either your dear ones go out, or stay in, but that ABSOLUTE QUIET is necessary while you rest. I remember being annoyed as a child that my grandmother insisted on this, but we were taught to respect her right to an afternoon rest.
What time do you get up and go to bed?
Last letters become first - March 26
Platonic friendships - do they exist?
What were your dream names for your kids when you were growing up?


but try telling that to anyone who has worked in a summer school and the students are away from their parents too.

