Snow Forts
Snow forts do not require any materials, other than snow. Your children can build without wood or nails. The only bad thing about a snow fort is that it does melt. In areas where a lot of snow falls, the kids can start their project in the nearest snow drift. The drift will probably be taller than they are, so all they really have to do is dig. Tell them to dig an entrance into the drift. Hollow out a big room and if they have time, make separate rooms. You can also suggest the room have windows. The fort will provide them with cover during snowball fights and the windows will give them a way to launch an attack from inside the fort.
Go Cheap
That big cardboard box your refrigerator came in can be used as a basis for your children's outdoor fort. This would be ideal for a summer fort. Small children can push the box on it's side and crawl inside. They can cut windows into the walls and draw flowers, trees and other decorations on the outside. This fort will be temporary as weather will destroy it.
Make It Sturdy
Your children can have a fort that will not deteriorate in the weather. Use the old building materials you have left over from previous projects. The wood for your deck can be used to construct a floor or roof. The sheets of plywood or aluminum can be used for the walls. For this project, your children will need extra help from you, particularly if sheets need to be cut.
Tree Fort
Tree forts are also called tree houses and have captivated children for years. They can climb trees, throw water balloons and play house at the same time. Choose a tree that is very strong and sturdy with strong branches. The design of the fort will depend largely on where the tree branches are placed. This fort will require even more involvement from you. Attach strong wooden boards to the tree for the floor of the fort. This is one aspect you should do yourself. Your children can help pick and set up the walls. Let their imaginations run wild. Build them a balcony. Attach a climbing rope, as well as a staircase for the kids who are not good climbers. Build more than one room. Instead of square, make it spherical or triangular.