SOLUTION: In our backyard, we have an old picnic table. If you know Aaron, you probably know that he doesn't enjoy the outdoors... so the picnic table is almost never used. Trying to convince Aaron to go have dinner at the picnic table and enjoy the evening outside pretty much never works. So I came up with another use for the picnic table.
I bought chalkboard paint at the Home Depot - around $4 for a spray can of paint. I covered the picnic table with the chalkboard paint and now the kids can draw with their sidewalk chalk while safely contained in the backyard! And if I should ever convince Aaron to come eat outside, we can just throw a tablecloth over the chalky picnic table.