Yeah for you getting out! Have a great time!!!
Ask your inlaws what they like, do they like to cook? Some people enjoy it and you could have the ingredients on hand.
I know you didn't ask about picky eating but do not plan meals around picky eaters. Kids shouldn't eat mac and cheese and chicken nuggets all the time because they refuse anything different. Now is a great age to get them to try new things and broaden the food spectrum. That doesn't mean every Friday can be mac and cheese and nugget night either. My kids get one or two nights a week they choose what we have. However the other nights it is up to me and they need to eat at least half of what I serve. They are allowed a few things each that I don't force them to eat, they tried it several times and don't like it, so I think that is fine for kids to have a few foods they just don't like.
Tell them whatever grandma and grandpa fix and serve is their meal and they will eat it or be hungry, period. I always remind my kids they do not have to eat what I serve, however do not ask for a snack later. Sounds harsh but catering to them especially while grandparents are watching them isn't good for anyone.
With new people watching them during meals, it may help them be willing to try new things. Kids tend to put up the biggest power struggle with parents. Do not have to have your inlaws cater to their picky eating either. They are watching the kids and it should be simple for them. Give your inlaws strict guidelines on if they do not eat or at least eat hafl of whatever they serve to not give the boys anything else. Kids will not starve, promise. My two cents on that one.
Aks them what they would like to do for meals, get the indgredients and special coffee or whatever they like to have at home there on hand.