Popcorn balls are a great ornament option for kids because they can help create them. To start, make popcorn balls. You can use this recipe from MarthaStewart.com.
Place the popcorn balls in cellophane bags and tie them with a piece of natural raffia, which you can find at floral supply stores. Hang the bags of cellophane from the tree using the raffia.
For an extra bit of elegance, drape the tree in wide green ribbon. A beautiful bow makes your tree look like a present in itself.