One of the best ways to check for understanding is to have your students relay their learned knowledge through creative writing. I use this creative writing activity as a formative grade for the World WAR I unit. I have students select a topic from the war and create a 20 page mini book highlighting the topic in a child friendly approach. I contact elementary school teachers and ask for their students to read the books upon completion. In the past I have even offered a Readers Theater to the younger grade levels, by having my 9th graders visit the elementary school and read their books to students.
In preparation of the books students have to write a rough draft for teacher review and peer review. Students then take suggestions and edits and create a final copy, including illustrations. I provide appropriate word choice for the selected grade level (F and Ps, site words, etc. etc.). It is also a good idea to provide students with the opportunity review children's books, picture books, etc. prior to writing (picture book showcase).
The elementary students could write questions back, create a mental picture from their book, or just enjoy listening to the high school students.