K.S.
I wouldn't spin it at all. Who says that everything adults get to do kids should get to do as well? There are plenty of great things that are only available to adults because that is just the way it is not because adults are cruel and are withholding things from kids.
My oldest wants lots of things DESPERATELY but we simply say wantin' isn't gettin'. Not to be cruel but because they are not the all-deserving-center-of-the-universe.
Just tell them you are going on vacation and they are getting a little mini-vacation of their own by staying with granma/grampa.