For younger kids I recommend the Peanut Butter and Jelly Sandwich algorithm. Bring in a loaf of bread, a jar of peanut butter, a jar of jelly, and a knife. Set them out on a desk and tell the class that you are going to be a computer and they are going to program you. They need to program you to make a PB&J. Tell them that as a computer you can only do what you are told, you don't know ANYTHING about making sandwiches. They will generally start out with "Open the bread", to which you reply, "What/Where is this bread you speak of?" You can get as specific or as general as you like, generally you can leave out things like how to undo the twist-tie on the bag, but when they tell you to spread the peanut butter you can really mash it in there and tear up the bread. It takes 15-30 minutes depending on the class to get an edible sandwich, but the kids love it.