There are certain activities that are just easy for children to have fun with and tire them out at the same time. Those are both amazing goals to have for your children: fun and tiring. The first activity is fishing. While this is not extremely strenuous, this activity allows for you to slow down the pace a little bit and take some time to truly interact with your kids. The fishing will give a great midday rest to your crazy camping time and slow down your heart rate a little bit. Plus, unless you are an excellent fisher, you and your child are pretty much on even footing when it comes to catching a fish. The second activity that you can take advantage of is swimming. This goes the other direction. This will tire you and your children out quite well. It doesn't have to be a pool or a nice beach at a lake. You can find tons of fishing spots in little ponds or a deep spot along the creek. Just check it out before your child goes trouncing in.
A great activity for your children may also be going on a hike. This works well because you can move at a good pace while your child runs ahead and then back - scouting out the trail ahead of you. It also gives your child a chance to look around and see the nature and perhaps some the local wildlife. But it definitely fulfills both requirements: fun and tiring. Check with the shelter at your campsite to see about other activities that the campgrounds may schedule for any given night or day.