I seriously suggest leaving them with Grandma for the weekend
You will enjoy yourself so much more!
But if that's not an option; crayons & coloring books, bubbles, hoola hoops, nature walks, dress up clothes, fairy wings & wands...
Things from home that will make them feel comfy in the tent, familiar stuffed animals and blankies, etc. I remember the hardest part of camping when my son was little was getting him to fall asleep in the tent, (not unzip the door and run off !)
Those nifty backpacks with "leashes" are excellent... especially if you have an escape artist on your hands.
Hope that helps