Wow! I'm surprised at some of the answers. We pretty much turned our kids loose in the campground by the time they were 13. They were equipped with their bikes and a walkie-talkie and they were required to touch base with me every hour and anytime they moved to another location within the campground. My kids never had an issue.
I would leave my kids at the camper with a cell phone. It's up to you whether you want to allow them to go to the pool or the lake while you're gone. I think I would rather be close by if they're going to be in water.