I was thinking more about this one. So disrespectful and unbelievable. I imagine they expect not to be confronted, but I'd be tempted to have this fantasy conversation:

Me: so tell me about the 2 McD's that day.

Them: oh it was so fun and it was right there and they love it.

Me: Ok. So you want to be free with their food while they're with you, right?

Them: yes.

Me: Ok. You know as the parent, I am responsible for their food and exercise, and their program. You know DS is obese, right, and failed every single one of his fitness tests? I got it in writing from the school.

Them: (what would they say? he's not obese? he didn't fail? )

Me: so let me get this straight. We have an obese and unfit child, and You want to do whatever even if it blows off his parent's program for him.

Them: well no we didn't mean that. We just think the parent's program needs to let grandparents do anything.

Me: It doesn't work like that. A day like you had can derail him and make him feel bad for days afterward. Do you want the details on this? or just the food rules? Maybe we can find a way that does both. But - What if you don't like what you have to do at McD's. eat only the meat and throw away the bun, the fries, the drinks, and dessert.