Locate a large piece of cardboard, taking into consideration the size you'd like your final tree to be. If you're making small 3D trees for tabletops, they'll need to be less than 12 inches. If you'd prefer to make one large tree, consider the size of space you have available. You need enough cardboard to cut the same tree shape twice.
Trace the shape of the tree on the cardboard with a pencil. You'll need to cut the tree out, so simple shapes work best. Draw a few large branches rather than attempting many small branches in order to make the tree look realistic and neat when cut out. Ensure that your tree has a wide trunk and a main branch that runs through the center of the tree, emerging at the top.
Cut the tree shape out with sturdy scissors. Once the shape has been cut, place it down on another piece of cardboard and trace around the shape, then cut out your new tree shape. This leaves you with two identical tree shapes cut from the cardboard.
Cut a slit at the base of the trunk of one of the trees and continue cutting until you reach halfway up the tree. Then, take the second tree shape and cut from the top of the shape, stopping when you reach halfway down.
Fit the tree shapes together by sliding the tree with the cut trunk into the tree with the cut top. Slide the trees together until they meet in the middle. You now have a tree that can stand up on its own with four distinct sides.
Cover the tree with brown butcher paper, if you wish.
Decorate the tree as you wish. You could have your preschoolers glue on colored leaves they've collected outside, or use the tree for learning. For instance, decorate the tree in construction paper leaves that you've labeled with letters. Then, have a child pick a leaf from the tree each day and discuss that letter together.