I would just talk it up as to how much fun it's going to be. We have gone a few kid free vacations (and they are amazing!!! good for you!!!) and it is hard because the kids don't totally understand. But what we do is just say how much fun they are going to have on Staycation with Grandma and make it sound exciting.
Not that you asked for it, but if you're interested, here's some other things we did to make it more exciting!
We bought some gift cards for our care givers to use to places like Chuck E. Cheese's, Build a Bear, Barnes and Noble, Sonic, etc. Not a ton of money but just so they could do a little something. We also left little envelopes for our kids to open each day. Inside was some sort of picture of the family or mom and dad for them to put inside a little photo album I made and a small present for each kid. It was little stuff, like $1 bin stuff or stuff I found on clearance. Sometimes I would put one of the gift cards in there so they could use it for lunch or a treat or something.
This was basically just a fun thing to A. make it super awesome and fun when mom and dad were out of town and B. give them a sort of calendar as to how many more days before we came home. We only do this if we are leaving them for a week or more…not just overnight or for a weekend.
We did call them every day but mostly they didn't care that we were calling! LOL!