I would say you either just tell them and don't worry about it, or you let someone park in the paring spot on your site.

Just my take on it, but...
Personally, I would just do the latter. You are staying in a campground, will a car parked in what amounts to a driveway in most cases, really matter that much to you? This is after all primarily a family reunion, not a bike tour.

If you care about promoting cycling, my guess is that you will be more likely to positively influence someone to consider cycling instead of using a car if you aren't seen as the jerk who wouldn't let us park in his empty parking space.