The only advice I would have is that since we are trying to keep it to 30 minutes, to maybe not give the kids the option to use the colored pencils that are in the bin. Since we had all those boxes of colored pencils, we gave the kids the option to color their own geometric shapes with the colored pencils or use the construction paper geometric shapes and glue them down. Many of the kids wanted to use the colored pencils, which is fine, but it just ended up taking longer, and then there were many pencils that broke, needed sharpening, etc.
Clean up was kind of messy. We put shapes and black strips on each table and let the kids pick what they wanted, so the ones that didn't get used ended up getting mixed up in the bags during clean up. However, I did use the paper cutter afterward to cut up more black strips and put them in their own bag and there is one good size bag of just geometric shapes too.
I brought a book with examples of realistic art and abstract art because I thought it would be helpful to have some more examples to show the difference and help them understand what is abstract art.