I'm in the camp that you shouldn't have to entertain kids all the time. My boys are four years apart and we never had to entertain them while camping. If we were camping for more than two nights, we would usually planned a day where we did some sightseeing but for a normal weekend of camping, they needed to entertain themselves. Don't get me wrong, we played games with them, ate meals as a family, etc. They always made new friends at every campground we went to.
Your 11 year old should be old enough to explore out of your sight with a friend provided she has a two-way radio and you know exactly where she will be. Set up check in times a close intervals and as you build trust/comfort, you can extend the check in times.
*edited to change "he" to "she"