This is an easy project for kids who are able to use scissors. I downloaded some leaf clip art and had the girls trace them onto paper of various fall colors. They then cut them out using safety scissors. Then we taped the leaves onto a piece of yarn and hung it on the curtain as a nice fall decoration.
For children too young to use scissors safely, you could have them color or paint the leaves and then you can cut them out. They'll be able to help you attach the leaves no problem!