You do not need to wait to teach free verse exercises. Free verse may be included at any point of your work with children. As with any other form it should be introduced in examples by reading to the class or having them read if they are comfortable with that. Have children write parts of conversations they have heard between adults or other children. Write poems that sound like the words of the teacher.
Many books give lists of ideas about which to write poems.
You can also find these on the Internet. If you can't find ideas, use any book of children's poems as an idea book for your own poems. Keep the first line and change all the other lines to make your own poem. Your copy will be autographed by the poet upon request.