Making trees can be rather complex. So we have developed a very simple process that has proven very popular with kids in group sessions at summer camps.
They were originally designed to be used with a small farm diorama, which diorama that you can find here.
The plans for small and large trees are available for download. And at the link to the download page that follows, you can download many other free plans. That link is Free Plans For Kids
This is a video showing how to make and use them.