Go on a nature walk and collect as many specimens of leaves as you can. Don't forget to get 'ugly' ones. Ones that have been eaten through or bitten off. Get a lot of pretty ones, in all colors, too. Let the child decide which ones go together.
Simple leaves are all one leaf like an elm or pin oak.
Compound leaves are made up of leaflets and look like they have lots of little leaves. Our mimosa tree is like this. See this site for help. Note other characteristics. We used a glue stick to hold the leaf down on a page and then covered it with clear contact paper.