To expand upon this, the kids create a matching book with a picture of their structure (and their written description complete with labels,etc). You could also use a story telling app--creating a digital book to document and write about their work.
Learning to Document in Reggio-Inspired Education article by Carol Anne Wien (Learning to document Reggio Emilia style. Helps you celebrate the stage you are at in LEARNING how to document in this way! Goes through 5 stages of learning how to document.
This post explains why documentation is important and a few different ways of documenting. Documentation allows you to see children's interests overtime and build off prior interests for future activities.