Many children are doing tree and leaf identification projects -- here are some great resources to help.
What Tree Is That is an online guide takes users through a step-by-step process of identifying leaf shapes.
Trees: Brain Boosters is a Kits for Kids set that includes many great resources for identifying trees.
Fifty Trees of Indiana is a CD-ROM compatible with Macintosh and Windows computers, published by Purdue University's Department of Forestry and Natural Resources.
Here are some other books you may find helpful:
101 Trees of Indiana: A Field Guide, by Marion Jackson
National Audubon Society First Field Guide: Trees
Trees of North America, by C. Frank Brockman