Poet Kenneth Koch once observed that young kids think of poetry as "something difficult and remote." But when he visited an elementary school in Manhattan, he saw "how playful and inventive children's talk sometimes was. They said things in fresh and surprising ways.... They enjoyed making works of art, drawings, paintings and collages."
Koch set out to discover whether kids could write poems in that same spontaneous and joyful manner. His success in the classroom is described in "Wishes, Lies and Dreams: Teaching Children to Write Poetry"—a mine of ideas for any parent who wants to encourage poetry writing at home. 
Koch asked each child to write one line of a poem. He shifted these together into one text—a mix of sense and nonsense. The kids loved the sound of it and it made them want to write more. He also asked each child to write a poem in which every line began, "I wish." The poems were "beautiful, lyrical, funny, touching—ways for kids to express exactly what they felt." 
The moral? As long as your kids feel free to be as playful as they like, and are asked to write about what matters, the results will be quite fantastic. Writing poetry—and hearing you read poetry aloud—are both ways to ignite a child's imagination.