Hey Wooly!
First off, for grandma and grandpa, they're not going to be able to get up and down off the ground easily. I strongly suggest some of the double height air mattresses or something to raise their bedding up off the floor. Also, remember that air mattresses will conduct cold, so when their bed is made, make sure you have a layer under them to keep them warm.... aka, make the bed with them laying on top of one thick sleeping bag, then another sleeping bag over them to keep them warm.
Your screened shelter is going to be great for if it rains. If you have a chance of severe storms and sideways rain, you can get a couple tarps at walmart and have those around the sides.
Since you're going to have electricity, a couple fans in case it gets hot. Also bring some folding camp chairs.
Entertainment - musical instruments, somebody bring a guitar, and a harmonica and y'all have fun. Some of the best music is just "musical doodling" Dominoes are great for camping, as are other board games like Scrabble. Something with plastic tiles that won't matter too much if they might get damp, or hand held card games like
Apples to Apples and
The Great Dalmuti For food, do not attempt to outdo yourselves cooking over a big campfire. We just got news that La Nina means we're going to have another dry winter. We've been fighting burn bans and wildfires all over the state. Breakfast is great when its simple. Some breakfast tacos of just eggs, cheese, potatoes and sausage, bacon, or ham. Supper is easy, but again, don't require a campfire to make it work, and keep it simple.