Brainstorming can be done by writing on post-its or by drawing especially when you need to come up with concepts ideas or when you work with kids. The first thing you need to tell the participants is that anyone can draw, they don't need to create a piece of art, simple shapes will be enough. It's about fast and "dirty". To show them their capabilities, you can use the method to Open the mind and don't forget about the Brainstorming rules that also apply here.
A good technique to use is called "Build on each other". Give each participant a piece of paper. Give them 2mins to draw as many ideas as they can. When time is up, they pass the paper to their right. The next rounds last 1min. When the paper is back to its first owner, it's over. The goal is to build on each other ideas or to start new ones. At the end, everyone should discuss about the different results and similar ideas can be grouped together.