If you pay the grandparents to watch the kids then you can have rules but if they are watching the kids out of the kindness of their heart let grandma be grandma.
If your son is picky premake and bring the food to grandmas so she just needs to pull it out of the fridge/freezer to heat or eat cold.
I know I tell my mother-in-law the simple outline of what we do during the day ex:
She sometimes eats breakfast, but usally just likes a smoothie or the drinkable yogurt.
she eats a small lunch around 11:30am usually but whenever you think she is ready to eat is fine.
she naps after lunch or sometime before 2pm... and sleeps for an hour or two. I am sure you are a good judge off when they are getting tired.
she usually likes a snack when is wakes up so whatever you want to give her.
... and so on.
Leave the timeline vague and let the grandparents have fun... they will not starve your children and your children willl eat when hungry (if you bring along food they like you will know grandma will have things on hand the kids will eat).
They more you stress out about the scheudle the kids will read that and be upset too if things are not like home. Tell the kids this is grandma and grandpas house and they will take great care of you... also tell them if they need anything just ask grandma or grandpa (including naps, snack, meal or soon on... that is what kids do). If the kids can talk now they will let grandma know what they want and if you tell it is ok to ask they will have a fun week at grandma's(they won't hurt grandma's feelings).